4i8 



BOTANY OF THE LIVING PLANT 



The rapidity of the spread of the disease is one of its most surprising 

 features. The fact that it habitually spreads down the prevailing 

 wind indicates that it is due to wind-borne conidia. The conidiophores 

 project through the stomata on the lower surface of the leaf and 

 branch repeatedly (Fig. 353). The end of each branch may swell 

 into an inverted pear-shaped conidium, which is constricted off from 



Fig. 353. 

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

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

 aerial branches which bear the conidia, h. 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 do^vnwards. (After Marshal] 

 Ward.) 



its very thin stalk. If growing in still air the first conidium may be 

 turned aside, the stalk growing on sympodially, and proceeding to 

 form a second conidium, and so on. Thus a succession of conidia 

 may be produced for a considerable time, each readily detachable, 

 and easily borne by the wind. 



Germination takes place only in presence of moisture. The proto- 

 plasm of the conidium divides into about ten parts, wliich escape 

 by rupture of its apex into llie water in the form of zoospores, very 



