The Algae - Ascomycetes - Hypocreales

Phylum Thallophyta - The Algae - Ascomycetes - Hypocreales 
The Hypocreales are Ascomycetes in which the asci are arranged in a perithecium which opens at the top by an ostiole. This perithecium is flask­shaped, and is either free or sunk in a stroma formed from the tissues of the Fungus. The asci usually produce eight ascospores which are generally hyaline, elliptical or filiform in shape, and may be composed of more than one cell. The sfroma, and the perithecia when free, are often brightly coloured, red, yellow or purple being common shades.
The group includes a number of interesting species, some of which are of economic importance. Species of the genus Nectria form bright red stromata on the surface of wood, pushing up and cracking the bark in the process. On account of their colour and appearance these Fungi are called the Coral Spots, and at least one species causes a canker disease of fruit trees. Other members of the group live as parasites, mainly of tropical insects, the bodies of which become filled with the fungal mycelium until the creature is killed. The peculiar appearance produced as a result of the growth of the Fungus out of the body of an insect gave rise in earlier times to a belief that they possessed medicinal and magical value. Cardyceps sinensis on caterpillars, for example, was a celebrated drug in the Chinese pharFacopoeia.
Consider one example of this order, Claviceps purpurea, which causes a disease known as Ergot of Rye. The Fungus contains a powerful muscular stimulant and is still used for medicinal yurposes.
Claviceps purpurea (Ergot of Rye) 
The Fungus, though characteristic of Rye, also attacks other species of grasses (Fig. 274), including Wheat and Oats. There are several biologic races: e.g., that growing on species of La/ium being incapable of infecting Rye and other cereals.
The ascospores are carried to the grass plants at the time when the flowers are maturing and find a lodgment among the floral parts. The hyphae formed from them penetrate and ramify in the ovary, where they form a thick mass of tissue, which becomes hard and purple on the exterior and forms a sclerotium. :\Iean­
while certain hyphae from within the tissues
of the ovary grow outwards through the ovary wall and their exposed apices function as
conidiophores and cut off small, spherical conidiospores. The effect of this fungal infection is to cause the enlargement
of the ovary, which at its upper end becomes rather spongy in character. During the development of the conidiospores a sugary secretion is pro­duced by the hyphae. Insects are attracted to this "honeydew" and aid in distributing the conidiospores to other flowers where they can produce fresh sclerotia.
SEXUAL REPRODUCTION
After the conidial stage has reached its climax the sclerotium begins to mature. The hyphae in the base of the ovary, instead of forming a loose weft of filaments, become compacted into a dense mass which grO\\'s in length and finally projects an inch or more from the ear of rye, which has by this time ripened, carrying at its apex the remains of the mycelium which bore the conidia. In this process the ovary is completely destroyed and the sclerotium occupies its place, lying between the glumes of the rye spikelet.
This sclerotium is called the ergot. It is a hard body with a purple or black outer coat and a white interior. In this form the Fungus passes the winter, usually in the ground when the flower stalk has withered. In the spring it begins to grow by producing a number of fleshy stalk-like projections, each of v"hich terminates in a globular head and is termed a stroma. These stromata are yellowish-brown in colour. When mature the globular heads are seen to be covered with slightly raised papillae, surrounding pores which are really the ostioles of perithecia. In a longitudinal section through the head these perithecia will appear as flask-shaped cavities arranged in a single layer all over the surface of the head.
The sex organs are formed in regular positions in the perithecial heads and produce a layer. Each originates from a single, elongated, multinucleated hypha \vhich is rich in cytoplasm and arises from a vegetative cell. The terminal cell of this branch swells and the nuclei in it divide and arrange themselves in pairs, forming a dicaryon. From the positions of the nuclei, branches arise, each of which is unicellular and each receives a single dicaryon. This branch then elongates considerably and nuclear division ensues. The branches then bend towards one another in pairs, one of a pair being stouter and thicker than the other, which is slender and elongated.
The former functions as an ascogonium, the latter as an antheridium. On the side of the ascogonium nearest to the antheridium, a papilla is formed, the wall dissolves and the nuclei of the antheridium pass into the tip of the ascogonium. The further development is uncertain, but apparently the nuclei pass.to the base of the ascogonium, while the tip dis­organizes and disappears. Later, in the position of the ascogonium, are seen binucleate cells, presumably derived from the ascogonium, which form ascogenous hyphae. The tips of these hyphae bend over, a. terminal cell is cut off, and from the penultimate cell an ascus is formed in which the ascospores develop.
Each perithecium contains a number of narrow, tubular asci which taper at either end. Each ascus encloses eight filiform ascospores. These ascospores are liberated through a small opening at the apex of the ascus. The asco­spores can be carried by wind to the flowers of the rye, which, if they are open, receive them. It has been recorded that the ascospores are them­selves capable of budding off conidiospores which will also infect the ovaries of rye plants. Thus the life-cycle is completed and we can represent it by the following diagram (Fig. 279) :-
ECONOMIC IMPORTANCE OF ERGOT 
The occurrence of ergot in rye bread may have senous consequences, causing gangrene and loss of hair, teeth and nails. Cattle eating ergot grain may show gangrenous and nervous symptoms \"ith the loss of hoofs, tails and horns, and miscarriage of young. Ergot poisoning, known as Ergotism, is very dangerous, the mortality being as high as 50 per cent.
Ergot is chiefly collected in Spain, Portugal, Poland and Russia, either by hand picking or by threshing through a special machine. Ergot contains Ergotamine and Ergotoxin, and is used medicinally to induce contraction of the smooth muscles, especially of the uterus in childbirth, and to control uterine haemorrhage.
Ergot contains, in addition to the above toxins, the substance acetyl choline, which causes muscular- contraction in dilutions as high as I part in 500 millions. Acetyl choline is responsible for normal muscular con­traction, being formed at the junction of the motor nerves and muscles, where it lasts for only a fraction of a second, producing contraction before it is destroyed by the enzyme cholinesterase. If its action is prolonged it causes paralysis.
Artificial infection of rye has been successfully accomplished in Vienna and also in Australia, though it still remains to be seen whether this can be done on a commercial scale.

The Algae - Ascomycetes - Saccharomycetales

Phylum Thallophyta - The Algae - Ascomycetes - Saccharomycetales 
The organisms included in this order are of somewhat diverse character, but they are all predominantly unicellular. They have probably originated from more than one group of Fungi and are now classed together because of their similarity of form. Some Fungi which are not at all closely related to this order can adopt under certain conditions a yeast-like habit of growth, so that the unicellular condition may have been arrived at along ore than one evolutionary line. It is at any rate reasonably certain that this feature is secondary or derived, so far as this group is concerned and not primitive in the evolutionary sense.
Some members of the order possess a definite sexual process, but in many it is wholly lacking. In the latter case reproduction is entirely vegetative. It may be that such species are really heterothallic, in which case the chances are very great against the two strains meeting to conjugate, except in artificial culture.
Saccharomyces cerevisiae (The Beer Yeast) 
The cells of Saccharomyces are very minute and either spherical or slightly oval. They are about 10u in diameter and are clothed with a delicate membrane, which appears to be a compound of two polysaccharides, glycogen and mannan, in combination with phosphoric acid. Cellulose is not present. There is abundant cytoplasm containing several small vacuoles in which are often grains of volutin (so called from its having been discovered in the bacterium Spirillumvolutans), which is ribo-nucleic acid or one of its salts.
During fermentation glycogen increases in the vacuoles so that they stain dark brown with Iodine. This carbohydrate is the chief metabolic product and reserve material of the yeast cell.
The method of nuclear division is still uncertain, some authorities maintaining that it is entirely amitotic, while others claim the existence of a simple form of mitosis with two chromosomes. The latter view seems the more probable in view of the life-cycle described below.
Species of yeast have been kept in cultivation by mankind since early times, and are widely used in fermentation processes, e.g., in beers, wines and milks, and for baking. Many species are, however, to be found wild on the surfaces of higher plants, especially on the fruits. When the latter drop in autumn the yeasts pass into the soil where they spend the ,,-inter. In spring they are splashed by rain back on to the plants, thus completing the cycle. We will deal with the chemistry of the fermentation of sugars by yeasts under Respiration in Plant Physiology.
ASEXUAL REPRODUCTION
Reproduction in Sacchawmyces takes place in two ways. The first is by the formation of buds on the vegetative cells, and the second is by means of ascospores, the true nature of which is open to question. Budding takes place very rapidly when the food supply is abundant and is in many species the only known method of multiplication. A bud begins as a small protrusion of the wall of the mother cell, usually near one end, which rapidly enlarges until it is as large as the mother cell itself. The junction of the
two cells is by a very narrow neck, and buds are therefore usually detached easily. Sometimes, however, when growth is most vigorous a bud will itself produce another bud or buds before it is detached, so that a short chain of cells is formed. It is not infrequent to find that the young cells remain attached to each other and to the parent cell for some time, so that quite large
groups may be found, containing up to a hundred cells, which remind us thai the yeasts, although normally unicellular, may be closely related to types of fungi which produce regular mycelia. When a bud is formed the nucleus of the cell divides and one portion passes into the bud, but whether this division is a mitotic process has not been fully established.
SEXUAL REPRODUCTION
Ascus formation occurs when the food supply, and especially when the water supply, is scanty. The commonest method of inducing the formation of asci is by so\ying an actiye culture on a damp plaster block in a closed glass dish. Another way is to spread the yeast on a sterilized slice of carrot which is kept dry. The asci are slightly enlarged vegetative cells, the contents of which divide and round off into four ascospores with rather dense and highly refractive walls.
When an ascus breaks down the four spores germinate, and now they fuse together in pairs with fusion of nuclei to produce two zygote, which start budding immediately and produce vegetative which will in time give rise to asci again. It sometimes happens, ho that an ascospore germinates without conjugation, and in this case it rise to much smaller, almost spherical cells, which also multiply by budding and form a " dwarf" strain. These cells cannot produce asci until two of them have conjugated to form a zygote, which by budding produces the normal, large-celled strain again. Frequently many such conjugations occur simultaneously among the dwarf cells so that it would appear as if the dwarf strain had almost wholly turned into the large-celled strain.
These facts find their explanation readily enough when viewed as an alternation of generations. The normal large-celled strain is diploid, i.e., is sporophytic, and gives rise to sporangia (asci) in which presumably meiosis occurs at spore formation. The ascospores would therefore, as in other cases, be monoploid. If they germinate singly they give rise to a small­celled gametophytic strain. This forms no definite sexual organs, but conjugation takes place, as in some other Ascomycetes, between vegetative cells, producing diploid zygotes, which reproduce the diploid strain once more. It happens, however, that in most cases the gametophyte phase is short-circuited, and the first cells produced from the germinating ascospores, instead of developing vegetatively, conjugate immediately to form a diploid zygote, so that the ascospores in such a case are the only monoploid cells in the life-cycle. whether there is any advantage in such a procedure, or whether it merely is the result of the propinquity of the ascospores we cannot say, but we know that such short-circuiting of an alternating cycle does in fact occur in very various types of organisms, e.g., in Puccinia among the Fungi; in Fucus among the Algae, and in the phenomenon of Apospory among Ferns.
The discovery of the conjugation between germinating ascospores in Yeast has opened up very remarkable and valuable possibilities of artificial hybridization, and already a considerable number of such hybrids have been created in the search for improved capabilities of fermentation.
Schizosaccharor.nyces octosporus 
We may contrast the account of Saccharomyces with the condition in another member of the Saccharomycetaceae, Schizosaccharomyces octosporus. In this species the vegetative cells are more or less spherical and, like all yeasts, are capable of multiplication by budding. Under certain conditions, how­ever, reproduction by a sexual process may occur. Two adjacent cells of similar size put out processes which come into contact and fuse, forming a conjugation tube. The nucleus from each cell passes into the conjugation tube and there they fuse together. The two conjugating cells now enlarge, and eventually a single, large oval structure is formed, which is an ascus. The zygote nucleus divides to form usually eight nuclei, around which ascospores become organized. Sometimes the remains of the conjugating cells can be distinguished after the formation of the ascospores, in which case four appear to lie in each of these cells.
In this species, therefore, it is clear that the ascus is a post-fertilization structure agreeing in the method of its production with that of a typical Ascomycete, and there is some evidence that meiosis occurs during the forma­tion of the ascospores. Hence in this type the true ascomycetaceous nature of the yeasts becomes more apparent.

The Algae - Ascomycetes - Phacidiales

Phylum Thallophyta - The Algae - Ascomycetes - Phacidiales 
The Phacidiales are Ascomycetes in which the apothecia are immersed in the fungal tissue which is partly buried in the host tissue. They are generally small in size and of a leathery or corky consistency. The group is a small one but includes several genera which produce spots on the leaves of higher plants. It should be noted, however, that many of the common leaf-spot diseases are not caused by members of this order. Some of these diseases are caused by members of the Sphaeriales in which the ascocarp is flask-shaped, whereas in the Phacidiales it is more or less flat and widely open. Others, and indeed the majority of leaf-spot Fungi, are placed among the Fungi Imperfecti because they reproduce only asexually, by conidiospores.
Consider one example of this order, Rhytisma acerinum.
Rhytisma acerinum (The Black Spot Disease of Maples) 
This Fungus is extremely common throughout the country, attacking the leaves of the common Field Maple and also the Sycamore; in fact, few trees and indeed few leaves escape. Superficially these leaves appear to have been spotted with large drops of tar, and it seems that the lower leaves are usually more frequently attacked and have larger spots than those at the top of the trees. These black spots occur on the upper surface and average about 15 mm. in diameter. They consist of a wrinkled stroma, roughly circular in shape, which is formed mainly in the epidermal cells of the leaf, though the hyphae spread down into the cells of the palisade layer as well.
The mycelium arises from a germinated ascospore which forms a germ tube and enters the leaf through the epidermis. It traverses the intercellular spaces of the palisade cells and also the spongy mesophyll.
In the epidermal cells the mycelium penetrates the cross walls repeatedly, so that the outer surface of the epidermal cells gradually becomes split away from the rest of the leaf. Meanwhile black material becomes deposited in between the epidermal cells, and the infected area changes colour from yellow to black.
ASEXUAL REPRODUCTION 
At a fairly early stage in development it is possible to see the elaboration of the future asexual reproductive area by the modification of the part between the upper and lower walls of the epidermal cells (Fig. 263). Not only does the deposit of black material become denser along the lower surface but the mycelium gradually forms a continuous tissue in the bases of the cells and produces vertically growing hyphae, which extend upwards towards the upper walls of the epidermal cells. These elongating hyphae are really conidiophores and later abstrict conidiospores.
Meanwhile the mycelium from which the conidiophores arise becomes more dense and forms a closely interwoven tissue of small uninucleate cells termed a plectenchyma. Kot only does this form the entire floor of the cavity, but circular ramparts grow up, dividing the area into a number of cavities which are for a time covered by the upper walls of the original epidermal cells, now completely disorganized. As these cavities enlarge vertically the conidiophores commence to cut off conidiospores in chains and fill the cavities, which eventually burst open by the breakdown of the upper epidermal walls. Such cavities, in which asexual conidia are produced, are termed acervuli. Despite the fact that these conidiospores are produced in vast numbers they have not been germinated under experimental conditions, and there is no evidence that they do actually spread the Fungus during the summer months when they are mainly produced.
SEXUAL REPRODUCTION 
The apothecia may be formed either in old acervuli or in newly formed cavities in the epidermis. During the autumn the plectenchyma becomes more sclerotic and blackens considerably. In the interior of the plectenchyma numerous apothecial cavities become differentiated. These become filled with loose hyphal tissue and are covered not only by the outer epidermal walls but also by a thick-\\'alled black tissue called the epithecium. Below, the cavities rest on a somewhat lighter, thick-walled tissue, the hypothecium, which in turn covers the lower epidermal walls.
In each of these apothecial cavities the female sex organs are formed.
Each consists of a uninucleate stalk cell, two or three multinucleate ascogonial cells and a uninucleate trichogyne. Apparently no male sex organ is formed and no true fertilization takes place. Instead, the septa between the ascogonial cells break down and their nuclei pair together.
Ascogenous hyphae now grow out from the ascogonium and paired nuclei migrate into them. The ascogenous hyphae branch repeatedly, and many asci are produced from a single ascogenous hypha. Each young ascus possesses a pair of nuclei derived by conjugate division from the original pair in the ascogenous hypha. These two nuclei then fuse.
The single fusion nucleus in the ascus now undergoes three divisions, resulting in the formation of eight nuclei, around which eight ascospores are formed. These ascospores are long and slender and lie side by side in a packet along the length of the ascus.
Meanwhile, from the loose hyphae in the apothecial cavity paraphyses are differentiated which grow up between the asci, so that at maturity the asci are more or less enveloped in a mass of very slender, elongated filaments.
The mechanism for bursting the apothecium is elaborate. Changes, mainly of the roof of the apothecial cavity, are initiated about the time that the asci begin to form. A layer of cells about three-quarters of the depth from the top break down, thus forming a small rift which gradually expands downwards, while at the same time the upper part of the roof rises and finally ruptures at the top. At this stage the apothecium is closed only by the tissue below the rift. When the spores are ripe this too ruptures and the apothecial cavity is exposed. Some apothecia never open widely and the opening remains as a narrow fissure, in others the roof bends back so that the whole cavity is exposed.
Since the development of the asci takes place during the winter it must obviously occur when the leaves have been shed, hence the discharge of the ascospores will take place in the early spring at a time ,yhen the leaf remains have largeiy rotted and the bits are blown about. In this way additional dis­tribution of the Fungus is effected quite apart from the fact that the ascospores are forcibly ejected from the ascus. Moreover, as a result of contraction of the epithecium the ascospores may be shot up as much as I mm. above the surface of the substratum. They ultimately reach the young leaves as they unfold i~ the spring and there germinate to start the life-cycle over again.

The Algae - Ascomycetes - Helvellales

Phylum Thallophyta - The Algae - Ascomycetes - Helvellales 
The Helvellales are large Ascomycetes usually growing saprophytically on dead wood, in which the fruiting body is fleshy and in which the hymenium is spread over the external surface of the greater part of hte fruiting body and covered at first by a membrane or veil. It seems probable that they have
originated by the eversion of an apothecium which was already provided with a stalk. By development on a convex surface the hymenial layer is increased in area, and this is further increased by folding of the surface which may in some genera result in the formation of a very complex, convoluted head supported on a sterile stalk. Stages in this evolution can be traced in this order and, to a lesser extent, in certain Pezizales, with the result that it is not easy to draw a sharp line between them, and some authorities prefer to merge the Helvellales into the Pezizales. This view has certainly much to recommend it.
Consider one example of the order, Helvella elastica, though in most features it differs but little, so far as is known, from a number of allied species.
Helvella elastica 
This Fungus occurs quite commonly in woods throughout Great Britain in company with a number of other species which differ mainly in colour and in the degree of fragmentation of the cap. Most of the species are edible and constitute the Morels of the mycophagist, though the true Morel (Morchella esculenta) belongs to, a distinct genus.
Helvella elastica is usually white, though sometimes tinted with pale pink or grey. It is clearly divisible into a stalk or stipe and a cap or pileus. The former is very firm, somewhat swollen below and deeply grooved. The cap is thin, somewhat lobed, with the lobes at first more or less attached to the stem, later becoming free. The Fungus occurs in moist places and is not uncommon in summer and autumn.
The mycelium consists of long, sparsely branched, interwoven hyphae, which show numerous anastomoses and may contain from two to many nuclei in each cell.
DEVELOPMENT OF THE FRUITING BODY 
Here and there, by more vigorous growth, little knots of hyphae appear the cells of which are thicker and shorter and the filaments more branched. These represent early stages in the formation of fruiting bodies.
Slightly later the main tissues of the fruiting body can be recognized, and in a knot of hyphae about o· 5 mm. in diameter it is possible to recognize a short thick stem passing insensibly into the mycelium below and surmounted by a bulbous cap, slightly larger in diameter than the stem. The whole structure is covered by a membrane. The membrane consists of two layers, an inner tissue termed the palisade layer, which is made up of the club-shaped ends of hyphae arranged close together to form an even surface. Outside this is an envelope formed from certain hyphae of the palisade layer which grow out beyond its surface, turn at right angles and grow over it. This envelope is transitory, breaking up at an early stage and then degenerating.
In the region of the cap paraphyses appear at quite an early stage. They force their way between the cells of the palisade layer, breaking the envelope as they do so. In this vvay a hymeniallayer becomes differentiated on the surface of the cap.
THE ASCOGENOUS HYPHAE 
So far as is known no sex organs are produced. At a fairly early stage, however, some of the hyphal threads form a mat of tissue a short way below the paraphyses. These cells are multinucleate and considerably thicker than the ordinary hyphae. It is from these cells that asci are produced by
the vertical growth of branches from this sub-hymeniallayer. The branches so formed divide repeatedly and penetrate between the paraphyses. The end cells of these branches contain deeply staining protoplasm and usually only two nuclei. These end cells curve over and form hooks, and in each hook the two nuclei divide into four. This is followed by the septation of the hook into a row of three cells. One nucleus enters the terminal cell, one the basal, and two remain in the middle cell. This middle cell enlarges and becomes the ascus, in which the two nuclei fuse. Meiosis of the fusion nucleus follows and by a further division eight nuclei are formed, around each of which an ascospore is organized.
These ascospores are oval in shape and brovvn in colour. They are ejected forcibly from the apex of the ascus, and occasionally a tiny cloud of spores may be observed around a fruiting body; when large numbers of ascospores are being expelled together.
It will be noted that the two nuclei which fuse in the ascus have been produced apogamously from vegetative hyphae and there is no evidence that any sexual apparatus is developed. By analogy with other types it may well be supposed that a hyphal fusion occurs somewhere prior to the production of a fruiting body, but work along this line has not yet been done.

The Algae - Ascomycetes - Pezizales

Phylum Thallophyta - The Algae - Ascomycetes - Pezizales 
The Pezizales are Ascomycetes in which the fruiting bodies have an outer covering which is either fleshy or leathery in texture. This definite external layer, or peridium, is closed at first, but subsequently is pushed open by the gro\yth of sterile hairs or paraphyses, forming an open, cup­shaped or discoid apothecium lined with asci.
The group includes many common Fungi which occur on dead and decay­ing wood. Many of the fruiting bodies are brightly coloured and are con­spicuous in woodlands during the autumn. Examples are Pyronema confiuens and Peziza vesiculosa.
Pyronema confluens (P. omphalodes) 
This little Ascomycete is sometimes found growing very abundantly in woods on burnt places, especially among half-charred masses of leaves, though it may also occur among damp well-decayed leaves even when there has been no fire. The mycelium is glistening white and resembles a frost-like tracery on the dark substratum on which it grows. It is on this mycelium that the sex organs are produced and only a few days are required for the development of these sex organs into mature fruiting bodies.
Pyronema confiuens has gained a reputation out of all proportion to its importance either ecologically or morphologically, on account of the amount of work which has been done on its sexual reproduction, for it is the only known member of the Pezizales in which a complete sexual apparatus exists. It may therefore be regarded as the starting-point of a series of types showing a degeneration through which the true function of the sex organs is lost, and finally the organs themselves, till the sexual act is reduced to the fusion of two nuclei within a cell \yhich is derived directly from the vegetative cells of the fruiting body, as we shall see in Peziza. As more and more of these simpler Pezizales are inyestigated more stages are supplied in this degenera­tion series, but so far Pyronellla confiuens stands alone in having the complete apparatus still present and functional. It is for this reason that we must consider the structure in some detail.
THE DEVELOPMENT OF THE SEX ORGANS 
The cells of the mycelium are multinucleate and form a system of threads which ramify through the substratum. Among these hyphae branching is rare, but at certain points one or more forks appear from which branches grow upwards and form a small tuft. These filaments then branch dichoto­mously, and the terminal cells of paired branches become differentiated into a functional antheridium and a female organ, the oogonium or ascogonium. Behind these are cut off one or two stalk cells, so that the antheridium forms a two- or three-celled branch, made up of uninucleate stalk cells and a multinucleate antheridium. The female branch consists similarly of one or two stalk cells, the swollen ascogonium and a trichogyne formed as a tubular process from the distal end of the ascogonium. Both the ascogonium and the trichogyne are multinucleate but unicellular.
As growth continues the nuclei of both the ascogonium and the antheridium increase in size, while those of the trichogyne remain small and finally disorganize.
FERTILIZATION 
The trichogyne now comes into contact with the antheridium, and the wall between becomes dissolved. The male nuclei which have united into a cluster in the centre of the antheridium now migrate into the trichogyne, whose own nuclei have meanwhile disappeared. At the same time the wall separating the trichogyne from the ascogonium disorganizes and the nuclei of the ascogonium become grouped at its centre, while the surrounding cytoplasm becomes increasingly vacuolated. The male nuclei now pass down the trichogyne into the ascogonium and become associated with the female nuclei.
,Vhether these male and female nuclei actually fuse in pairs or whether they simply remain associated together is a matter of dispute. According to some workers there is definite nuclear fusion so that the resulting nuclei are diploid. Others consider that the male and female nuclei associate but do not fuse, so that they remain monoploid and subsequently divide in pairs (conjugate division).
THE ASCOGEKOUS HYPHAE 
After a few hours changes have taken place in the fruiting body. In the first place the ascogonium has become enveloped in a tissue of vegetative hyphae which have originated from the lower cells of the original knot of tissue, thus forming the peridium. At the same time outgrowths have been formed from the wall of the ascogonium, and into each of these a pair of nuclei has made Its way. These are the ascogenous hyphae. They branch repeatedly, and by conjugate division each branch is supplied with a pair of nuclei or dicaryon. Septa now appear, cutting the hypha into a number of cells, each with one dicaryon in it. Finally the terminal cells bend oyer to form a hook like an inverted V, known as the crozier, and from the penultimate cell, which forms the apex of the hook, there grows out upwards a single elongated cell, the ascus.
The two nuclei of the dicaryon in the young ascus fuse. This fusion in the ascus is immediately followed by three nuclear divisions whereby eight nuclei are formed. It will be noted that there is no question about the nuclear fusion at this stage, hence it follows that if a nuclear fusion also did occur in the ascogonium then the fusion nucleus in the ascus must be tetra­ploid, seeing that it is formed as the result of two nuclear fusions. Alterna­tively, if there was only an association of male and female nuclei in the ascogonium and the sexual act was only consummated in the ascus, then the nucleus will be a typical diploid. Now, since obviously the nuclei of the ascospores must be monoploid, as they give rise to a monoploid mycelium, it follows that the normal procedure of meiosis will not provide the mechanism for reducing the chromosome complement of a tetraploid to a monoploid. It is postulated therefore by those who hold the view of a double fusion that there is correspondingly a double reduction, and that this second nuclear reduction takes place in the third division in the ascus. The name brachy­meiosis is given to this division.
In either case the mature ascus contains eight nuclei around each of which a "'all is laid down, resulting in the formation of eight separate ascospores which are smooth and ellipsoidal.
The asci arising from each group of ascogonia are arranged side by side in a concave cluster or disc, around which the peridium forms a marginal layer, thus constituting an open ascocarp or apothecium.
In the meantime the sex organs themselves have disorganized and hyphae grow in from the arvthecial sheath to form a layer of paraphyses lining the inner surface of the apothecium. As the asci mature they push their way between these paraphyses so that at maturity tbe lining layer of the apothecium, the hymenium, is composed of an intermingled mass of asci and paraphyses. As this fruiting body matures it flattens out and the colour of the hyphae composing the hymenium changes from clear \yhite through pale pink to salmon red, thus rendering it a very pretty little structure up to about 3 mm. in diameter. Since the sex organs are frequently produced quite close together on the mycelium, it follows that the mature fruiting bodies tend to run into one another, and Pyronema confiuens, a8 the name implies, is generally found with its fruiting bodies in confluent masses.
Peziza vesiculosa 
This Fungus is to be found growing on manure piles and on heavily manured soil, and on account of its large size is readily recognized. The cups are often irregular in shape and may become contorted (Fig.
vegetative mycelium ramifies within the substratum on which it lives, form­ing a very complex system, through which food is absorbed by the fungus. The fruiting bodies either arise directly on the substratum, when they are said to be sessile, or they may be elevated on a short stalk. Externally they are white or buff in colour, but the inside of the cup is brown. The outer surface is frequently covered by minute wart-like pustules, but the inside is smooth.
ASEXUAL REPRODUCTION 
This type of reproduction occurs irregularly by the production of conidiospores, or by the formation of thick-walled spores, termed chlamydo­spores, which develop singly or in groups within the cells of the mycelium. Both germinate to produce fresh hyphae.
SEXUAL REPRODUCTION 
Sexual reproduction in the Pezizales varies considerably. A series of types showing progressive reduction in the sex organs can be seen by a comparison of species. Firstly it appears the antheridium ceases to function, as we have already seen in certain species of Aspergillus. Then the trichogyne of the oogonium disappears, and finally the oogonium itself is not formed, but in its place certain cells of the vegetative hyphae take over its function and all that is left of the original sexual process is the fusion of two nuclei within a purely vegetative cell.
In Peziza vesiculosa the apothecium begins as a weft of tangled hyphae, and in the centre of this mass are certain cells which are said to possess nuclei which fuse in pairs. These cells then give rise to ascogenous hyphae. From these the asci are formed, and in them the ascospores are produced. There exists in this species the same question as to whether there are two fusions, the one of the paired nuclei in those vegetative cells which play the part of the sex organs, and the second fusion in the ascogenous hyphae, or whether there is only the second one. According to which view is adopted the last division of the nuclei in the formation of the ascospores will either be regarded as a reduction division or not. The ascospores are arranged obliquely in a row within the ascus, and each is ellipsoidal and smooth in outline.
The asci stand side by side forming a continuous layer over the inner surface of the apothecium. They are cylindrical and are intermingled with straight, slender hyphae, which are termed paraphyses. The special layer formed of the asci together with the paraphyses is termed the hymenium. The eight ascospores are discharged from the ascus through a terminal aperture, only under damp conditions. Breathing on the surface of a ripe fruiting body will often cause the liberation of a cloud of spores. On germination they either produce a fresh mycelium, or they may give rise to a hypha from which conidia are developed.

The Algae - Ascomycetes - Erysiphales

Phyllum Thallophyta - The Algae - Ascomycetes - Erysiphales 
The Erysiphales are Ascomycetes which are characterized by an abundant mycelium,
which is often dark coloured. The sex organs are enclosed within a tissue termed the perithecium, which is without an ostiole. The asci are formed together in a group from a regular layer in the base of the perithecium.
Many of them cause the White or Powdery Mildews or Blights on flower­ing plants, where they live as parasites on the leaves. They are almost cosmopolitan in distribution, but have been most commonly found in Europe and North America.
Consider two examples of this order, Sphaerotheca humuli and Erysiphe polygoni because of the marked difference in the methods of sexual reproduction.
Sphaerotheca humuli (The Hop Mildew) 
This Fungus is the cause of Hop Mildew or Mould and is one of the most serious diseases of the Hop. It chiefly affects the leaves and the young strobili, rendering the latter useless. The intensity of the disease varies from season to season and is best controlled by dusting the hop bines with finely powdered sulphur.
The life-cycle begins with the germination of either the asexual conidio­spore or the sexual ascospore upon the surface of the host plant. A series of hyphae are formed which spread over the surface of the leaf, producing a white web-like coating with uninucleate cells. From these hyphae slender branches penetrate the cuticle and grow down into the epidermal cells, form­ing ramifying branches within the cells by which food is absorbed. These special cells are termed haustoria, and are the only part of the Fungus which enters into the host plant. Penetration by the haustoria is secured in a peculiar manner. The mycelial hyphae send out short branches which apply themselves very closely to the epidermis of the host and spread out into an adhesive disc called an appressorium. From the centre of this disc appears a new and extremely fine outgrowth which pushes through the cuticle like a needle-point. On entering the cell this outgrowth enlarges into a normal-sized hypha and branches freely.
ASEXUAL REPRODUCTION 
During the summer months reproduction is asexual by means of conidio­spores. The conidiophores are short, upright, unbranched hyphae which abstrict from their upper ends single chains of large oval conidiospores.
SEXUAL REPRODUCTION 
Sexual reproduction in Sphaerotheca hu.muli occurs in the late summer by the formation of perithecia, which are spherical structures 0.05 to 0.3 mm. in diameter and furnished with simple appendages. The perithecium is formed by a mass of hyphae which aggregate into a tiny ball. It is white at first, but as it matures it turns clear yellow, and finally brown.
The sexual organs arise as lateral branches from the mycelium. The ascogonium arises from a cell which divides to form a stalk cell and an oogonial cell, the latter being uninucleate. The antheridium is smaller and is also borne on a narrow stalk cell. It also contains a single nucleus. Antheridium and ascogonium stand side by side in close apposition. The wall between the two sex organs breaks down, and the male nucleus enters the ascogonium and unites with the female nucleus. Nuclear division follows and the zygote cell divides into a row of cells, the penultimate cell of \yhich contains two nuclei. This row of cells is equivalent to an ascogenous hypha. From the penultimate cell the ascus is produced. \Yhile this is going on, hyphae form around the developing sex organs, producing the wall of the perithecium. The two nuclei in each ascus then fuse, and immediately the zygote nucleus divides into four, during which reduction occurs. This is followed by a further mitotic division which, according to some workers, is also reductional. Thus eight nuclei are formed in each ascus and around them walls are formed producing the eight ascospores.
It will be seen from this account that there may be two fusions, one in the ascogonium itself and a second in the ascogenous hypha, and hence, to bring the chromosome number back to the monoploid, two reduction divisions are necessary. According to many workers a nuclear fusion does not occur in the ascogonium, but there is merely an association of nuclei, and they consider that the only true fusion occurs in the ascogenous hypha. If this is the case there is only one reduction division, namely the first division of the nucleus in the ascus.
During development the wall of the perithecium becomes differentiated into inner and outer layers, which can be easily distinguished. The inner layer is made up of thin-walled cells, rich in protoplasm. It forms a packing round the developing asci and supplies them with food. The function of the outer layer is for protection. Its cells have scanty contents and their walls undergo a change analogous to lignification.
From special cells of this layer the characteristic filamentous appendages are differentiated. These appendages consist of long, single-celled outgrowths which show various degrees of complexity in different genera. Their shapes are quite characteristic and are chiefly used as a basis of the separation of the genera.
The life-cycle of Sphaerotheca humuli is represented in the following diagram.
In the classification of the Powdery Mildews we have a clear and simple example of the principles of plant classification, and it may be worth while to mention it briefly here. The genera fall naturally into two groups; firstly those with only one ascus in each perithecium, and secondly those in \yhich more than one ascus is produced. In the former section there are two genera:
Sphaerotheca, in which the appendages are long and hyphal-like, and Podo­sphaera, in which the ends of the appendages are repeatedly and dichotomously branched. In the second group are four common genera: Erysiphe, in which the appendages are like those of Sphaerotheca; Microsphaera, in \yhich they closely resemble Podosphaera; Uncinula, in which the tips of the appendages curl inwards; and, finally, Phyllactinia, in which they are straight and stiff with a swelling at the base. The following diagram brings out these points clearly.
Erysiphe polygoni (The Pea Mildew) 
This species causes a Mildew of Peas and also of Swedes and Turnips.
In the former case it may attack the pods as well as the leaves. Swedes and Turnips are often attacked in a dry summer to such an extent that the leaves are turned completely white by the mycelium.
The Fungus spreads oyer the surface of the host plant, sending down fine haustoria into the epidermal cells where they spread out forming absorptive organs. During the summer months reproduction is by means of conidiophores which abstrict conidiospores in chains. These are entirely similar to those in Sphaerotheca both in their development and form, and we need not consider them here. In fact it is only in the method of formation of the sexual reproductive organs that this species displays an interesting and important difference.
SEXUAL REPRODUCTION 
Initially there is a conjugation between short branches of two adjacent hyphae, with the result that a fusion cell is formed. There is no evidence that any nuclear fusion occurs, though the nucleus of one hypha passes into the other. This stage is followed by formation of additional cells, which are budded off the fusion cell, or central cell. The cross walls of these cells are not closed, and the whole structure forms a coenocyte or maze, as it is sometimes called. Meanwhile, additional tissue grows up surrounding this maze and forming the inner and possibly the outer wall of the perithecium.
After a while the cells of the maze break down to form a cavity in the centre of the perithecium, while by the formation of additional septa the cells of the perithecium, which were originally multinucleate, become progressively uninucleate.
From near the base of the perithecium there then grow up into the cavity, several cells each of which is binucleate. Their nuclei fuse, and in this way a number of asci are produced. The fusion nuclei divide meiotically, followed by a second and third division to form the eight nuclei of the ascospores, around each of which the spore wall is ultimately formed.
When mature the whole perithecium is blown away by the wind, and may become attached to a fresh leaf by the appendages. Here the wall of the perithecium breaks down and the asci burst, liberating th.e ascospores, which germinate, forming fresh hyphae. It is generally found that the ascospores are not formed until the spring, since the perithecium oyer­winters on the ground among dead grass, development stopping in the autumn at the formation of the asci.
BIOLOGIC RACES 
Recent work has shown that in Erysiphe graminis there are several biologic races, that is to say that different strains exist, which are morphologically indistinguishable but among which it can be shown that the strain \yhich attacks one species of grass will not infect any other grass. Similarly, cultivated cereals each have their own particular strain. A point of some interest is the existence of bridging species among the host plants of the biologic strains of E. graminis. Thus the strain which infects Bromus racemosus will not attack B. commutatus directly, but it will attack B. hordeaceus, and an infection from the latter species will attack B. commutatus. No explanation is forthcoming as to what advantage the fungus deriyes from this very specialized restriction of possible host plants. We shall, however, meet with the same specialization in Puccinia graminis.
Similar biologic races occur in the types we have described. In Sphaerotheca humuli there is a biologic form which occurs on the cultivated strawberry, while in the United States there is a form which attacks roses. In Erysiphe polygoni the form which attacks Peas will not affect either Swedes or Turnips, while only the race on the latter plant is able to attack other species of Brassica, especially Kale. In England, however, this latter form rarely produces perithecia.

The Algae - Ascomycetes - Plectascales

Phylum Thallophyta - The Algae - Ascomycetes - Plectascales 
The Plectascales are Ascomycetes in which the product of the fertilization of the ascogonium is frequently only a single ascus. In some of the higher members, where more than a single ascus is produced, the asci are not arranged in any system, but are scattered irregularly in a special closed body termed the perithecium.
Consider one example of this order, Aspergillus.
Aspergillus (Eurotium) herbariorum 
This Fungus is a saprophyte on fatty substances and forms one of the most frequent contaminants of food. Some species are parasitic on man and animals, but the majority live as saprophytes. The mycelium consists of filaments of multinucleate cells, and ramifies over the surface of the sub­stratum on which it lives, sending up vertical hyphae ,yhich bear the asexual reproductive organs. The name Eurotium was originally applied to the sexual stage, while the name Aspergillus was used for the asexual. This was due to the fact that until comparatively recently the two stages were regarded as those of distinct Fungi, owing to the fact that either stage could apparently continue indefinitely without the appearance of the alternative stage. This tendency among Fungi for the asexual stage to continue generation after generation without the intervention of a sexual stage we have already met with in Mucor. It is equally true of many of the Ascomycetes, in fact there exists a large group of Fungi, termed the Fungi Imperfecti, which includes many important plant pathogens, and comprises Fungi in which only the asexual stage is known.
ASEXUAL REPRODUCTION 
The asexual reproductive bodies are borne on conidiophores These arise as vertical hyphae which grow out from the mycelium. They are thick and are not divided by septa. The tips of these conidiophores
become swollen and bud out numerous small processes, the sterigmata, from each of which conidiospores are produced in chains in acropetal succession. The spores are distributed by air currents, and on coming to rest on a suitable substratum each germinates to produce a hypha from which a fresh mycelium develops.
SEXUAL REPRODUCTION 
The archicarp or fruit primordium consists of three parts: a multicellular stalk, an elongated unicellular ascogonium, and a unicellular trichogyne. The antheridium arises from the same initial hypha as the 42.-Aspergillus herbariorum. A to D, Stages in the development of the perithecium around the sex organs. E, Section through perithecium showing ascogenous hyphae. F, A single ascus containing ascospores. (After de Bary.) ascogonium, or from an adjacent one. It is unicellular and multinucleate. Fusion takes place between the antheridium and the trichogyne, which become twisted around one another during the process. It has not been proved, however, that the antheridium is ever functional in fertilization. In some species the antheridium may be absent, but this does not prevent the further development of the ascogonium. In either case the ascogonium becomes divided into a number of binucleate cells, from each of which a septate ascogenous hypha is given off. From the terminal cell of each hypha or from the cell below, termed the penultimate cell, an ascus is formed by elongation. In this cell the two nuclei fuse together.
Shortly after the development of the sex organs, hyphae begin to grow up around them and the whole structure becomes enclosed in a t\yo-layered envelope, the perithecium, the inner layer of which forms a nutritive tissue, while the outer serves for protection. This latter tissue secretes a bright yellow substance ·which makes the sexual reproductive bodies easily recognizable.
After the fusion of nuclei \yithin the ascus the zygote nucleus so formed divides meiotically into four, and this is followed by a further mitotic division. Around each of these eight nuclei a wall is laid down forming an ascospore. These spores have a sculptured epispore wall and are rounded in shape. They are liberated upon the decay of the perithecium surrounding the reproductiw organs. They germinate to produce fresh mycelium.
Closely allied to Aspergillus is the genus Penicillium, which is used, associated with other Fungi, in the "ripening" of cheese. It differs chiefly from Aspergillus in the form of the conidiophore. The conidiophores are narro,,· and branched and not swollen at their tips, while the coni­diospores are abstricted in more or less parallel chains. The sexual reproduction is similar to that of Aspergillus. The well­known antibiotic substance Penicillin is a product of Penicillium notatum.

The Algae - Phycomycetes - Mucorales

 Phyllum Thallophyta - The Algae - Phycomycetes - Mucorales 
The l\Iucorales are Phycomycetes in which the sex organs are relatively simple, and in which sexual reproduction may only occur at very infrequent intervals. When it takes place it consists of the fusion of two non-motile gametes. In most forms, including Mucor, these are morphologically identical, but in some other types they differ in size. Asexual reproduction is by means of aerially distributed spores, which may either be enclosed in a sporangium or may be abstricted in chains from conidiophores.
Consider only one example of this order, Mucor.
Mucor mucedo (The Common Bread Mould or Pin Mould)
This Fungus lives exclusively as a saprophyte and is frequently found producing a whitish-grey down on bread, cheese, jam, and other food material. It is also very widely distributed in the humus of the soil. It may be cultivated by keeping damp horse-dung or bread under a bell-jar for a few days.
The mycelium of Mucor consists of long, slender hyphae, generally white or colourless, which ramify over the surface of the food material and send down absorptive hyphae into it, for it is by this means that the essential nourishment is taken into the mycelium. These hyphae are coenocytic, though cross walls may occur at intervals in old hyphae.
The Fungus grows very rapidly, covering the substance on which it is feeding. Concurrently with growth there is abundant provision for repro­duction. Normally this is effected asexually by means of spores, though sexual reproduction occurs very occasionally and under special conditions, which we shall discuss later.
ASEXUAL REPRODUCTION 
Within a few days of the appearance of the mycelium asexual reproduction begins. Vertical hyphae (sporangiophores) grow out from the mycelium, which are destined to produce terminal sporangia. The sporangium is formed by the enlargement of the apex of the hypha into a spherical vesicle containing many nuclei. The protoplasm of the sporangium is divided by cleavage planes into uninucleate masses, from each of \yhich an ellipsoidal spore is formed. By a similar cleavage plane a dome-shaped septum is formed in the basal part of the sporangium, called the columella, which separates the body of the sporangium from the sporangiophore. The wall of the sporangium hardens and becomes brittle, and the entire sporangium darkens in colour until it is almost black when mature. The wall is covered with a deposit of small crystals of calcium oxalate. Kot all species of Mucor produce terminal sporangia. In some they may be developed on branched hyphae or in clusters. It is on such characters as these that the species are chiefly separated.
vVhen mature the sporangium wall is surrounded by a drop of water, secreted by the sporangiophore, and breaks round the base, leaving a ragged collar attached to the sporangiophore. The spores are not immediately shed but remain clinging together in a wet mass, and the means of their dispersal is uncertain. It has been claimed that insects are the chief agents, but it seems probable that eventually, when the whole mycelium has died and dried up, that some spores at least are scattered in the air. Their wide dispersal, especially in the soil, is othenyise difficult to understand. Spores germinate immediately on reaching a suitable substratum and produce a hypha from which a new mycelium develops.
SEXUAL REPROUCTION
It is well known that zygospores are only rarely found in cultures of Mucor, but for long the reason for this was not understood. It was shown by Blakeslee in 190+ that the mycelium of the Fungus is unisexual and not capable of producing both male and female gametes. Sexual reproduction, therefore, can only occur when different mycelia intermix. Two distinct
types exist, which are called" plus» and" minus» respectively. Zygospores can only be formed when mycelia of both types meet. There is no morpho­logical distinction between the mycelia, though obviously there must be a physiological one. Such a condition, where the thalli are morphologically alike but physiologically different, is termed heterothallism. It is widespread in the Fungi, and many species are found to exhibit this phenomenon.
When the hyphae of the two strains come into contact, short lateral branches, called progametangia, are developed, which adhere together by their tips. They swell and a transverse septum appears in
each, cutting off a terminal cell or gametangium. The remainder of the progametangium is known as the suspensor. These gametangia are multinucleate, and when mature the wall between them breaks down and a single fusion cell is formed. The wall of the fusion cell becomes thickened and dark coloured, and develops a rough warty surface. This cell is now termed the zygospore. The thick wall forms the exospore, while within it is a thin membrane called the endospore. The suspensors wither and the zygospore is set free.
Various interpretations of the nuclear behaviour inside the zygospore have been suggested, but owing to the very small size of the nuclei there is no agreement bet\yeen the views expressed. It may be that there is general pairing of the nuclei, so that a number of diploid nuclei are produced, or, as seems more likely, that only one nucleus from each gametangium actually fuses.
The zygospore retains its vitality for a considerable time and undoubtedly will resist unfavourable conditions. Since, however, zygospore formation is rare, it must not be regarded as the chief method of tiding the Fungus over bad conditions, for the asexual spores appear perfectly capable of doing this. When germination of the zygospore occurs the wall splits open and a single hypha grows out which terminates in a sporangium. Considerable importance is attached by some to this hypha, which is called a prornycelial hypha, and to the sporangium which it produces. Howeyer, the structure of the sporangium differs in no way from the typical sporangium produced asexually, and the spores produced in it give rise to fresh mycelia in due course.
No definite information is available as to the position of meiosis, though various workers have studied the problem. According to some the zygote
nucleus divides into four prior to germination, a condition analogous with what occurs in Spirogyra. Many suggest that reduction division occurs in the sporangium borne on the promycelial hypha. It is on account of this latter view that the hypha produced by the zygospore assumes special importance for, if reduction division occurs in the sporangium it produces, this hypha must be diploid, in contrast to the rest of the mycelium. Moreover, the sporangium formed on it must also be regarded as distinct, since, unlike asexual sporangia, it is the seat of meiosis. In the light of our present knowledge, however, we cannot say where meiosis occurs, nor are we sure that there is actually any nuclear fusion in the zygospore, though it is reasonable to assume that there is. If so, then at some subsequent stage there must be a meiosis to bring back the nuclei to a monoploid state.
As in the previous genera we can trace the gradual evolution of a conidio­spore from the sporangium of Mucor. The conidiospore condition seems to have been reached by the gradual reduction in the number of spores produced in a sporangium until there is only one, when the body may be truly regarded as a conidiospore, since it germinates by a germ tube.

HETEROTHALLISM 

The phenomenon of heterothallism is so important in the Fungi that we must say something more about it. Though the process was first discovered in the :\Iucorales by Blakeslee in 1904, it has now been shown to occur in many families of Fungi. In many species there is nothing in the structure of the mycelia to distinguish them from one another, but where they meet a line of zygospores is formed. In such cases it is customary to speak of one strain as plus ( + ) and the other as minus ( - ). 

In a few species of Mucor it has been found that zygospores are produced by the interaction of hyphae in a mycelium produced from one spore, and such species are said to be homothallic. In some instances imperfect hybridization may occur between opposite strains of different heterothallic species and between both (+) and (-) strains of heterothallic species on the one hand and homothallic species on the other. In such cases only the early stages are gone through and the mature zygospore is never produced. 

In certain other genera the gametangia of homothallic species are either not quite alike, or they may even differ markedly from one another. Such a species is said to be a heterogamic homothallic one. One such example is Absidia spinosa in whicG the gametangia are of different sizes. Here the larger is regarded as equivalent to the female and the smaller to the male gamete. It has been found possible to produce a hybrid between Absidia spinosa and 1VIucor hiemalis, which has identical gametangia, that is, it is homogamic but heterothallic. As a result of such a cross it has been discovered that the (-) strain of Mucor hiemalis united with the larger gametangium of Absidia spinosa, while the ( +) strain of M. hiemalis united with the smaller gametangium. It is therefore concluded that the ( + ) strain of a heterothallic homogamic species is the female strain, while the ( - ) strain is male. It may be mentioned that, as a result of biochemical tests, these conclusions of Blakeslee have been substantiated. 

The Algae - Phycomycetes - Peronosporales

Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales 
The Peronosporales are Phycomycetes in which the vegetatiye mycelium is composed of branched coenocytic hyphae, septa occurring rarely in older hyphae or in the formation of the sex organs. Sexual reproduction is oogamous, but only one oosphere is formed in each oogonium. Fertilization is effected by a fertilization tube. Asexual reproduction may be by sporangia, which either give rise to zoospores or produce spores which germinate by a germ tube. In the higher members conidiophores are produced which abstrict conidiospores, which also germinate directly by a germ tube. Intermediate stages between the two types are known. Where zoosporangia are produced the contents pass out into a vesicle in an incompletely diff­entiated state and then divide into reniform zoospores with lateral flagella.
The species are characteristically parasitic on higher plants, though a few occur as parasites of Thallophytes. Some can live as saprophytes after the death of the host, but in the higher members this faculty is absent, nor can they be cultivated on artificial media.
Consider three examples of this order, Pythium , Cystopus and Peronospota , each of which represents a separate family and shows an apparent advance on the previous one in adaptation to a land habit.

THE EVOLUTION OF THE CONIDIOSPORE

CONIDIOSPORE EVOLUTION 
While it is obviously unwise to draw too rigid conclusions about the evolution of a structure from a study of a limited number of existing types, the genus Pythium provides a number of examples in which the asexual reproductive organs apparently show a transition from a simple zoosporangium to a true conidiospore. It must be realized, however, that in citing the following species as stages in such a series the reproductive organs may in certain instances germinate by different methods under differing conditions, and that many species of Pythium exhibit both zoospores and conidiospores, the former being produced under wet and the latter under dry conditions.
In the genus Phytophthora, which is included in the family Pythiaceae, the zoosporangia develop from the vegetative mycelium and form zoospores which are kidney shaped and escape directly from the sporangium without the formation of a vesicle. Such a condition is not found in any species of Pythium, for this is one of the characters used to separate the genera. In Pythium gracile, however, the zoosporangia are filamentous and differ but little from the ordinary hyphae. Each, however, produces a long, narrow vesicle which terminates in a spherical swelling and resembles a zoospor­angium in appearance. It is this s\yollen sporangium-like structure which grows out of the host cell and in \yhich the zoospores are developed, and from which they are finally liberated as reniform, biflagellated bodies.
In Pythium debaryanum, as we have seen, the zoosporangia are spherical in shape and develop a vesicle into which the contents of the zoosporangium pass and in which the zoospores are differentiated. Under dry conditions, however, conidiospores are formed which are similar in shape and size to the zoosporangia. In fact, we may consider that these conidiospores are really zoosporangia which form no zoospores but germinate by a germ tube.
In Pythium intermedium both zoosporangia and conidiospores occur. The latter germinate by one or more germ tubes, while the former produce zoospores in a vesicle. In both cases, however, these reproductive bodies are formed in chains so that the conidiospores appear to be abstricted in a similar way to the zoosporangia of Cystopus, but they do not give rise to zoospores as in that genus.
In Pythium vexans zoosporangia are rarely formed, but the asexual repro­ductive organs behave as conidi,ospores, the germ tube being produced in place of the papilla of a zoosporangium.
Finally in Pythium ultimum zoosporangia are never formed, but spherical terminal conidiospores are- produced which germinate by a germ tube in the usual way.
A similar series can be traced in the genera which comprise the Perono­sporaceae, and we may briefly outline the series here.
In the genus Basidiophora the sporangia are borne in clusters from the top of a clavate sporangiophore. Each zoosporangium is spherical and possesses a definite apical papilla through which biflagellated, reniform zoospores ultimately emerge.
In the genus Plasmopara the sporangiophore is complexly branched and the zoosporangia develop terminally on the ultimate branches. Usually the contents of the zoosporangium divide up into separate zoospores which escape, but sometimes the structure forms a germ tube and behaves as a conidiospore.
In the genus Bremia the sporangiophores are complexly branched, each branch terminating in a saucer-shaped expansion bearing a number of sterig­mata. On each of these a single conidiospore is borne. This conidiospore possesses an apical papilla, and it is through this that the germ tube emerges. Zoospores are never formed.
Finally in the genus Peronospora itself the branched sporangiophores terminate in long, fine sterigmata, each of which bears a single conidiospore. There is no apical papilla, and the germ tube is produced from an indeter­minate point on the side of the spore.
From the above evidence, therefore, it seems reasonable to consider that the conidiospore, at least in this group, has been derived from a zoo­sporangium, the contents of which produce a germ tube instead of dividing up into a number of separate zoospores. This change from motile zoospores to conidiospores may have been brought about as an - adaptation to drier conditions in which motile zoospores were less successful than aerially distributed conidiospores.

The Algae - Phycomycetes - Peronosporales - Pythium debaryanum (The" Damping-off" Fungus)

Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Pythium debaryanum (The" Damping-off" Fungus) 
Unlike Saprolegnia, Pythium debaryanum can live as a parasite within the tissues of seedlings, causing them to collapse and producing a diseasewhich is frequently spoken of as " damping-off". During the early stages in the growth of the Fungus no external signs of its presence are notice­able on the host, and it is only after it has gained a firm hold that the hypocotyl collapses, with the appearance of having been pinched off near the base. With the death of the host the Fungus continues to live saprophytically upon
the dead remains, and by means of external hyphae and zoospores attacks other seedlings near by. The disease spreads rapidly, and in a comparatively short time large numbers of seedling plants are killed. Pythium debaryanum also lives as a saprophyte in the soil, remaining there indefinitely before once again becoming parasitic on some higher plant. It should be noted that many other species of Pythium also cause disease among higher plants.
Pythium consists of long branched coenocytic hyphae, transverse septa being absent, except in association with the sex organs. The contents of the hyphae consist of a vacuolated mass of protoplasm in which nuclei are embedded. The infecting hypha may find its way into the host through a stoma, or may gain an entrance by dissolving the cell wall, probably by enzyme action. Once inside it grows rapidly, branching and forming an extensive mycelium. Although for the most part the hyphae remain in the inter­cellular spaces of the stem, they have the power of penetrating into the cells.
sually the mycelium is restricted, at any rate in the early stages of the disease, to the cortical region of the hypocotyl just above soil level ; the cells of the host die and collapse at this point and the seedling falls over. Later the hyphae make their way into the vascular bundles, where they absorb food substances which are being carried to different parts of the plant .. It is the appropriation of this food and the formation of waste products by the Fungus which lead to the death of the host and so cause the Fungus to become saprophytic. Long before the death of the seedling, however, reproductive organs are developed. Reproduction may be either by asexual or sexual methods.
ASEXUAL REPRODUCTION 
The asexual reproductive organs are formed almost exclusively on aerial hyphae. Such hyphae are produced from a mycelium which grows out from inside the host, in the presence of a damp atmosphere, which favours growth
of the Fungus and also the production of zoospores. The tip of a hypha swells up and develops into a zoosporangium, which is then cut off by a septum. When the zoosporangium is mature a papilla is formed either at the apex or from the side of the sporangium. It is at first tubular, but later swells out at its distal end to form a vesicle. Into this the contents of the sporangium pass in a partly differentiated state, and in the papilla
zoospores are formed. They are reniform with two lateral flagella. The vesicle then breaks and the zoospores swim away in the water film. It some­times happens that the sporangium functions as a conidiospore, and instead of forming zoospores grows out directly into a germ tube which produces a m:w hypha. In this way a new mycelium develops without the intervention of swimming spores. This provision enables reproduction to take place under relatively dry conditions where there is insufficient moisture for zoospores to swim in.
SEXUAL REPRODUCTION 
The sex organs are developed within the tissues of the host, and consist of antheridia and oogonia. They appear at a late stage in the life­history of the Fungus, when the food material of the host is nearly exhausted. The oogonium is developed from the tip of a hypha into which some of the protoplasm, together with a number of nuclei, migrate and are cut off by a septum. The tip now becomes svvollen, and the protoplasm separates into a central granular portion, the oosphere, and a peripheral layer, the periplasm.
All except one of the included nuclei migrate to the periphery and abort, so that only one functional female nucleus remains in the centre.
Meanwhile the antheridium is developed as a club-shaped branch, either on the same or on an adjacent hypha, and is also cut off by a transverse wall. The antheridium now elongates, its tip comes into contact with the wall of the oogonium, which it dissolves at this point, and a fertilization tube is formed which penetrates the oogonium and passes into the oosphere. It is probable that only a single male nucleus enters the oogonium, and this immediately fuses with the female nucleus. The fertilized oogonium secretes a thick wall round itself, and thus becomes an oospore. In the oospore are a large number of oil globules \yhich constitute the food resen'e of the zygote. The oospore is a drought-resisting structure, associated with the dessication which the Fungus may experience. The germination of the oospore only takes place in the presence of moisture and may be delayed
almost indefinitely. The thick wall of the oospore bursts, and the contents, surrounded by a thin membrane, may either grow out directly into a hypha, \yhich immediately forms a fresh mycelium, or, under other conditions, this hypha may bear an asexual sporangium. Occasionally, however, the develop­ment of the hypha is suppressed, and zoospores similar to those which would have been borne in an asexual sporangium are developed directly within the oospore. These swim about in the water surrounding the host, until they find fresh tissues to penetrate, when they form a germ tube which develops into a hypha.
The life-cycle of Pythium is graphically represented thus ;-
It is important to realize that a large number of species of Pythium occur in this country, many of them in the soil, and that 'vvhile some are destructive parasites of higher plants others may play an equally important part in the decomposition of plant remains in the soil. These species show a consider­able nriation from the type described above. For example, in some the oogonium may be beset with spines, while in others the chief differences are in the formation of the zoosporangia and the germination of the zoospores. In fact it is possible to trace certain evolutionary sequences indicating a possible gradual evolution from a typically aquatic mode of life to one adapted to terrestrial conditions among the various species of the genus. A similar sequence is found in the genus Peronospora, and we will postpone our account of these matters till after we have described the life-history of this organism.

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The Algae - Phycomycetes - Peronosporales -Peronospora (The Downy Mildew)

Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Peronospora (The Downy Mildew)
There are a number of species of Peronospofa which occur on higher plants, all of which cause diseases known as the Downy Mildews. Most of these species occur on distinct host plants and differ from one another only in minor particulars of the size of the spores. Recent investigations have tended to split up certain of the older species. Thus the well-known form Peronospora parasitica, which is widely distributed on Cruciferous plants, has been split up into a number of separate species. One of the commonest forms of this is Peronospom cheiranthi, which occurs on wallflowers.
It lives as an obligate parasite within the tissues of its host plant and spreads by means of a branched mycelium, which consists of hyphae that ramify between the cells of the host. These hyphae do not penetrate into the host cells but only pass between them. They give off, however, lateral branches which enter the cells of the host and inside form branched or swollen structures which are called haustoria. These secrete enzymes, by the aid of which the protoplasm of the host cells is broken down and absorbed by the Fungus. Externally the Fungus causes a soft, white, downy appearance on the leaves of the host plant, often associated with twisting and stunting of the affected area.
Peronospora parasitica is commonly found on Capsella bursa-pastoris (Shep­herd's Purse), frequently associated with Cystopus candidus (White Rust). Other species are known to attack many crops of economic importance, such as beet, maize, tobacco and turnips. A Fungus which, like Peronospora, only attacks a specific host is said to exhibit selective parasitism.
ASEXUAL REPRODUCTION 
Peronospom reproduces both asexually and sexually. The asexual repro­duction shows an advance on the condition found in Pythium, in that the reproductive bodies are specially adapted for aerial distribution. The organs are termed conidiophores, on which are borne specialized conidiospores. The conidiophores arise from the intercellular mycelium, emerge through the pores of the stomata, and become repeatedly and dichotomously branched. They may be to some extent septate. To the end of each of the branches there passes a single nucleus; a swelling occurs, and into this the nucleus passes. The swelling increases until a spherical or oval spore is formed, attached to the conidiophore by a short thin stalk or sterigma, which is water soluble, a fact which facilitates distribution of the spores in damp weather.
Only a very small proportion of the conidiospores produced by the Fungus ever come to rest on a suitable substratum, i.e., the leaf of another plant of the same species of host, under environmental conditions suitable for germination. The Fungus must therefore produce a very large number of spores on the chance that a few will find conditions where germination is possible. The presence of a water-soluble sterigma tends to ensure that spores shall only be shed in weather when the humidity is sufficiently high to render germination possible.
Under favourable conditions for germination the spore wall bursts and a germ tube is put out, which grows over the surface of the leaf until it comes to the pore of a stoma, when it immediately passes down into the leaf and forms the mycelium which, as we have already said, ramifies in the spaces
between the cells. Considerable investigation has been directed to the problem of what induces the growth of the germ tube into the stomatal opening and the growth of the young conidiophore out of it at a later stage. Chemotropism and hydrotropism have both been suggested, but the true explanation is still a matter of some uncertainty.
SEXUAL REPRODUCTION 
Sexual reproduction is of an oogamous type, resembling in general that of Pythium. It takes place under conditions adverse to the Fungus and in the deeper layers of the host tissue, generally in the stem, and especially in the pith. The sex organs consist of antheridia and oogonia, and are formed terminally at the apices of either the same or of separate hypha. The oogonium is a swollen structure containing, at an early stage, a large quantity of cytoplasm and many nuclei. The antheridium is more tubular in shape, and it, too, is multinucleate.
As the oogonium swells a separation into a dense central oosphere and a less dense periplasm occurs, associated with changes in the position of the nuclei. Up to this stage they have been more or less evenly distributed through the oogonium, but with these changes in the cytoplasm one nucleus places itself centrally in the oosphere and functions as the female nucleus, while the rest pass into the peri plasm and eventually abort. The antheridium now comes into contact \yith the oogonium, and a fine fertilization tube is formed which penetrates through the periplasm and discharges a single male nucleus, together \yith a small amount of cytoplasm, into the oosphere. Conjugation of male and female nuclei takes place and a zygote is formed. Fertilization stimulates the secretion of wall material which occurs in the periplasm. The precise way in which this takes place varies in different species. In general, it appears that within the oogonial wall a very thick wall is laid down, forming the wall of an oospore, and enclosing the original oosphere. The periplasm may also contribute an exospore wall to the oospore.
The oospore is a resting spore, remaining unchanged until the decay of the host tissues, and after that it may remain in the soil for some months before germination takes place. Under favourable conditions it germinates by the bursting of the wall and the development of a germ tube. In cases where the Fungus parasitizes an annual plant the oospore is the means whereby the Fungus passes the winter months after the death of the host, and germina­tion takes place the following spring. There is some doubt as to precisely where the reduction division occurs.
Some authorities claim that it takes place during the multiplication of nuclei in the formation of the oogonium and antheridium, while according to others it occurs in the first division of the germinated oospore, and on the whole this Control of Downy Mildew and other Mildews is generally effected by covering the plants with some substance which will kill the Fungus without damaging the host tissues. In recent years great advances have been made in the study of sprays and spraying machinery. Two chief types of sprays are employed, the one in \yhich a substance or substances are dissolved in liquid and sprayed on to the foliage with the idea of covering the whole surface of the host with a film of the solution. The second is by the use of extremely fine powders which are blown over the plant, relying on the damp­ness of the leaves to retain them. In the former case a great proportion of the liquid is lost, and what is known as a " spreader" is now employed to increase adhesion of the spray to the leaves. This may take the form of an oil, the whole solution being worked up into an emulsion. Where Downy Mildews are concerned the solution known as Bordeaux mixture is generally employed. It consists of a dilute solution of copper sulphate and lime, sometimes with the addition of paraffin oil.

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The Algae - Phycomycetes - Peronosporales - Cystopus candidus (Albugo candida) (The White Rust of Crucifers)

Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Cystopus candidus (Albugo candida) (The White Rust of Crucifers) 
This Fungus attacks all kinds of cruciferous plants and may sometimes cause swelling and twisting of the stems, especially in young plants. The disease is easily recognizable by the white, smooth, glossy patches which gradually turn powdery and give the Fungus its common name of White Rust.
The fungal mycelium is intercellular, like that of Peronospora, and sends haustoria into the cells of the host plant; these haustoria are fairly well developed and end in button-like projections in the host cells. It is through them alone that the Fungus obtains its nourishment. It spreads rapidly within the host and may attack all parts of the plant, besides those where the white patches are apparent. Reproduction is both sexual and asexual, though there is no alternation of generations.
ASEXUAL REPRODUCTION 
The asexual reproductive organs are developed just below the epidermis of the stem or leaf of the host plant. In this position short sporangiophores are produced in compact groups, forming an almost complete tissue. As growth continues a nucleus passes to the tip of each sporangiophore, and this is followed by a deepening constriction around the tip, thus cutting off a zoo sporangium with one nucleus. Immediately behind, another 200­sporangium is cut off in the same way, until chains of sporangia have been formed, one above the other, from each sporangiophore. The outer layer of the wall of the sporangium becomes gelatinous, and as a result the sporangia tend to stick to one another.
The growing pressure of all these 200 sporangia cut off below the epidermis finally causes it to lift up, then to split and break up, exposing the white sporangia. It is interesting to note that during the development of the sporangia they are protected by the host epidermis, which is only ruptured when at last the distal sporangia are mature.
The zoosporangia are finally liberated and disseminated by the wind, and when conditions are favourable, that is, when the atmosphere is damp, they germinate by producing a number of zoospores (Fig. 229) in each zoosporangium. These zoospores are kidney-shaped with two laterally placed flagella. From these zoospores fresh infections of host plants are derived.
It is sometimes stated that Cystopus reproduces asexually by means of conidiospores. According to this view the sporangiophore is regarded as a conidiophore, from the tip of which conidiospores are abstricted in acropetal succession, in a way similar to that found in many Fungi, e.g, Aspergillus. It is certainly true that the method of abstriction of the zoosporangia in Cystopus is effected in much the same way. The difference lies in the method of germination of these zoosporangia. The typical conidiospore germinates by means of a germ tube and thus forms a hypha directly, one hypha being normally formed by each conidiospore. In Cystopus, as we have seen, the zoosporangium gives rise on germination to a number of zoospores, which are not only actively motile but capable of affecting a separate infection of the host tissue. It follows that the structure producing these zoospores must be regarded as a zoosporangium, comparable with that of Pythium.
From this vve may conclude that the typical conidiospore is equivalent to a
zoosporangium in which the contents do not divide into separate zoospores but germinate by a germ tube. In Cystopus we see how one stage in the process of conidiospore formation may have been evolved by abstricting these zoosporangia in chains.
SEXUAL REPRODUCTION 
The sex organs are developed in the intercellular' spaces of the host tissue and very often in the deeper layers of the stem. They consist of oogonia and antheridia. The oogonium is globose and is developed from a terminal, or occasionally from an intercalary swelling of a hypha. The mature oogonium may contain as many as a hundred nuclei, together with a quantity of food material. It is finally cut off by a septum from the rest of the hypha. The antheridium is club-shaped and is developed in close association with the oogonium. It is also multinucleate, containing some twelve nuclei, and it is also separated from the supporting hypha by a septum. At this stage the nuclei in both the oogonium and antheridium simultaneously undergo one nuclear division.
At about the time when the antheridium comes into contact with the oogonium the protoplasm of the latter undergoes differentiation, leaving a zone of rather translucent periplasm surrounding the denser oosphere, which contains most of the nuclei. At the point of contact of the antheridium and the oogonium the antheridial wall becomes very thin and the oogonium pushes its way in, forming a slight protuberance, which is called the receptive spot. It appears to be functionless, for shortly afterwards there develops from the antheridium a fertilization tube, which penetrates the receptive spot and enters the oosphere. All the nuclei in the antheridium and the oosphere now undergo a second nuclear division, while the nuclei in the peri plasm disorganize. A single female nucleus passes to the centre of the oospore wall oosphere and remains there, while the others pass to the periphery and finally abort. A single nucleus from the antheridium, together with a small amount of cytoplasm, passes in through the fertilization tube and the male and female nuclei come into contact and fuse. The fertilization tube collapses and is withdrawn from the oosphere, leaving a large vacuole. The wall of the oospore, thus formed, is now laid down. It is a thick, two-Iayered structure consisting of a thin endospore and a thick, often almost spiny, exospore.
Inside the oospore the zygote nucleus begins to divide, resulting in the formation of about thirty nuclei, after which activity temporarily ceases, and the oospore may remain for a long time in this condition. Later \\"hen germination occurs the wall splits and about a hundred zoospores are liberated. It is obvious, therefore, that prior to germination further nuclear divisions must take place, and though it has never been conclusively demon­strated, it appears likely that each of the thirty nuclei found in the oospore after fusion undergoes t\\"o divisions, which involve a meiosis, so that the nuclei of the zoospores are monoploid.
The life-history of Cystopus candidus may be represented by the following diagram (Fig. 232) :-
The Fungus is not a serious disease of cultivated crucifers, for though it frequently attacks them, it is rarely that the host plant is seriously affected. The oospore can apparently retain its vitality for as much as four years. The infection of the host takes place only through the cotyledons of the seedling or through such delicate tissue as the flower buds. The parasite cannot penetrate the mature epidermis, consequently an epidemic spread of the disease among crucifers is not very likely.
Despite the fact that Cystopus was the first name given to this fungus, and therefore undoubtedly the correct one, American workers have for many years used the name Albugo. The confusion is increased by the fact that the genus is now included in a separate family of the Peronosporales under the name Albuginaceae, in which the type genus is, according to European workers, Cystopus.