FIG. 72. Section of Potato leaf, showiiig development of mildew, Perono- 

 spora infestans. The figure shows the mycelium, o, interlaced among the 

 cells of the potato leaf ; the stalk, or hypha, 6, passing through a stoma ; 

 and the gonidia, c, c, which bear the spores. (After Marshall.) 



FIG. 73. Surface of Potato leaf greatly magnified, showing germinating 

 spore entering breathing pore at a, and going through epidermis at c. 

 Another spore that has not begun to germinate is seen at 6. (Ward.) 



131 



