osition moderately lighted or
protected by a blackened bell, the conidia very readily produced
zoospores.

A second form of germination of the conidia in _P. infestans_, when
sown upon a humid body or on the surface of a drop of water, consists
in the conidium emitting from its summit a simple tube, the extremity
of which swells itself into the form of an oval vesicle, drawing to
itself, little by little, all the protoplasm contained in the
conidium. Then it isolates itself from the germ-tube by a septum, and
takes all the essential characteristics of the parent conidium. This
secondary conidium can sometimes engender a third cellule by a similar
process. These secondary and tertiary productions have equally the
character of sporangia. When they are plunged into water, the ordinary
production of zoospores takes place.

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 p