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.
