408 



BOTANY OF THE LIVING PLANT 



(Fig. 308). Coming then to rest, the cilia are dropped: each zoo- 

 spore rounds itself off, and, investing itself with a wall, puts out a 

 hyphal tube. If this takes place on the surface of a potato leaf, as it 

 well might do under conditions of rain or heavy dew, all is ready 

 for the infection. This may be either by entry through the pore of a 



Fig. 307. 



Section of Potato-leaf, in the tissues of which is the mycelum of Phytophthora. 

 The hyphae run between the cells and send out through the stomata, a, c, d, the 

 aerial branches which bear the sporangia, b. The dark parts of the tissue of the leaf 

 show where the cells are dying from the effects of the parasite. Highly magnified. 

 The normally upper surface of the leaf is here turned downwards. / is a glandular 

 hair of the Potato. (After Marshall Ward.) 



stoma or by direct perforation through the epidermal wall (Fig. 309). 

 By either route the germ-tube of the parasite may reach the inter- 

 cellular spaces and establish a new infection. 



It is not only the leaves but also the stems and tubers of the 

 Potato-plant that may be traversed. The mycelium spreads through 

 the tissues down the haulms to the tubers. Young tubers may be 

 infected by way of the " eyes ", the lenticels or through wounds. 

 Tubers thus infected may decay at once under moist conditions 

 and when other saprophytic organisms are present, otherwise a 

 dry-rot results. Tubers in the latter condition may be among 



