Fungi: Their Nature and Uses

by M. C. Cooke

Lastly, there is a third mode of germination which the conidia of P. infestans manifest, and which consists in the conidium emitting from its summit a simple or branched germ-tube. This grows in a similar manner to the conidia first named as of such species as P. effusa. The conditions which control this form of germination cannot be indicated, since some conidia which germinate after this manner will sometimes be found mixed with others, the majority of which furnish zoospores. It may be that the conidia themselves are in some sort of abnormal condition.

In all the species examined the conidia possess the power of germination from the moment of their maturity. The younger they are the more freely they germinate. They can retain this power for some days or weeks, provided they are not entirely dried. Dessication in an ordinary temperature seemed sufficient to destroy the faculty of germinating in twenty-four hours, when the conidia had been removed from the leaves on which they were produced. They none of them retained the faculty during a few months, hence they cannot preserve it during the winter.

The germs of Peronospora enter the foster plant if the spores are sown upon a part suitable for the development of the parasite. It is easy to convince one’s self that the mycelium, springing from the penetrating germs, soon takes all the characters that are found in the adult state. Besides, when cultivated for some time, conidiiphorous branches can be seen growing, identical with those to which it owes its origin. Such cultivation is so readily accomplished that it can be made upon cut leaves preserved fresh in a moist atmosphere.

In the species of Peronospora that inhabit perennial plants, or annual plants that last through the winter, the mycelium hidden in the tissues of the foster-plant lasts with it. In the spring it recommences vegetation, and emits its branches into the newly-formed organs of its host, there to fructify. The Peronospora of the potato is thus perennial by means of its mycelium contained in the browned tissue of the diseased tubers. When in the spring a diseased potato begins to grow, the mycelium rises in the stalk, and soon betrays itself by blackish spots. The parasites can fructify abundantly on these little stalks, and in consequence propagate themselves in the new season by the conidia coming from the vivacious mycelium.

The diseased tubers of the potato always contain the mycelium of P. infestans, which never fructifies there as long as the skin of the tuber is intact. But when, in cutting the tuber, the parenchyma occupied by the mycelium is exposed to the contact of the air, it covers itself with conidia-bearing branches at the end of from twenty-four to forty-eight hours. Analogous results are obtained with the stalks of the potato. It is evident that in these experiments nothing is changed except the contact of the air; the specific conditions particularly remain the same. It appears, therefore, that it is this contact alone which determines generally the production of the conidiiferous branches.[M]

The mode of germination and development in the Mucors has been studied by several observers, but most recently by Van Tieghem and Le Monnier.[N] In one of the common forms, the Mucor phycomyces of some authors, and the Phycomyces nitens of others, the process is given in detail. In this species germination will not take place in ordinary water, but it readily takes place in orange juice and other media. The spore loses colour, swells, and absorbs fluid around it until double its original size and ovoid. Then a thick thread is emitted from one or both extremities, which elongates and becomes branched in a pinnate manner. Sometimes the exospore is ruptured and detached loosely from the germinating spore. After about forty-eight hours from the first sowing, the mycelium will send branches into the air, which again become abundantly branched; other short submerged branches will also remain simple, or have tuft-like ramifications, each terminating in a point, so as to bristle with spiny hairs. In two or three days abruptly swollen branches, of a club shape, will make their appearance on the threads both in the air and in the fluid. Sometimes these branches are prolonged into an equal number of sporangia-bearing threads, but most frequently they divide first at their swollen summits into numerous branches, of which usually one, sometimes two or three, develop into sporangia-bearing threads, while the rest are short, pointed, and form a tuft of rootlets. Sometimes these rootlets reduce themselves to one or more rounded protuberances towards the base of the sporangia-bearing threads.

Fig. 93.—Zygospores of Mucor phycomyces. (Van Tieghem.)

There are often also a certain number of the branches which had acquired a clavate shape, and do not erect themselves above the surface, instead of producing a fertile thread, which would seem to have been their first intention, become abruptly attenuated, and are merely prolonged into a mycelial filament. Although in other species chlamydospores are formed in such places on the mycelium, nothing of the kind has been traced in this species, more than here indicated. Occasionally, when germination is arrested prematurely, certain portions of the hyphæ, in which the protoplasm maintains its vitality, become partitioned off. This may be interpreted as a tendency towards the formation of chlamydospores, but there is no condensation of protoplasm, or investiture with a special membrane. Later on this isolated protoplasm is gradually altered, separating into somewhat regular ovoid or fusiform granules, which have, to a certain extent, the appearance of spores in an ascus, but they seem to be incapable of germination.