reserved 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 br