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]