The conidia of _P. umbelliferarum_ and _P. infestans_[L] show an analogous structure. These bodies, if their development be normal, become zoosporangia. When they are sown upon water, one sees at the end of some hours the protoplasm divided by very fine lines, and each of the parts furnished with a small central vacuole. Then the papilla of the conidium disappears. In its place appears a rounded opening, by which the parts of the protoplasm are expelled rapidly, one after the other. Each of these, when free, immediately takes the form of a perfect zoospore, and commences to agitate itself. In a few moments the sporangium is empty and the spores disappear from the field of the microscope.
The zoospores are oval or semi-oval, and in _P. infestans_ the two cilia spring from the same point on the inferior border of the vacuole. Their number in a sporangium are from six to sixteen in _P. infestans_, and from six to fourteen in _P. umbelliferarum_. The movement of the zoospores ceases at the end of from fifteen to thirty minutes. They become motionless, cover themselves with a membrane of cellulose, and push out slender bent germ-tubes which are rarely branched. It is but seldom that two tubes proceed from the same spore. The same development of the zoospores in _P. infestans_ is favoured by the exclusion of the light. Placed in a position 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.