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. umbellife- 

 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 



