GERMINATION AND GROWTH. 155 
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 six- 
teen in P. infestans, and from six to fourteen in P. wmbellife- 
rarum. 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 de-. 
velopment 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 pro- 
duced 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 pro- 
toplasm contained in the conidium. Then it isolates itself from 
the germ-tube by a septum, and takes all the essential character- 
istics 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 pro- 
duction 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 
