140 



ing growing tubers. This is accounted for by the 

 zoospore dying before it comes in contact with the 

 tuber. A zoospore may germinate on the surface of 

 the ground, but as it does not live many minutes, 



Fig. 18 Germination of zoospores of Phytophthora on epidermis of 

 potato. At a the germ-tube is entering a stoma ; at c it bores directly 

 through the cell-wall. Very highly magnified. Marshall Ward. 



before it is washed down to the tuber it may be dead. 

 When the zoospore has worked its way through the 

 outer tissue of the leaf it comes into contact with the 

 living cells, from which it at once commences to 

 extract food, and it rapidly grows and spreads about 



