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.
