Host and Parasite in certain Diseases of Plants. 439 



FiG. 14. Zoospores of a species 'of Pythium allowed to germinate in water on a 

 piece of a longitudinal section of a bean- stem. The zoospores (a) soon come 

 to rest, and one was noticed at mid -day on an exposed cell (b), lying nearly 

 over its nucleus. At 12.30 this spore had begun to germinate and enter 

 the cell (c). At 2.5 p.m. the germinal hypha had turned to the left, and com- 

 menced to bore through the side-wall with its tip (d). At e is shown the 

 progress by 5.30 p.m. on the same day ; and at f (smaller scale) the condition of 

 affairs at 12.30 next day. That theseihyphse pierce the walls by means of secreted 

 zymases can scarcely be doubted after what has been proyed for Botrytis. 



