174 



LEAF AND SEEDLING DISEASES 



to live till the spring and then to send out a small germ 

 tube of limited growth, called the promycelium, and this 

 bears a few, generally four, very small spores, which are 

 known as sporidia. The sporidia are very light, and being 

 lifted by the wind settle on the leaves of neighbouring 

 plants. But, though they may germinate, they inevitably 

 perish unless they happen to come in contact with a plant 



FIG. 71. 



of some definite species, the host B. In the case of the 

 black rust of wheat, this plant is the barberry. The germ- 

 tube which grows from a sporidium pierces the cuticle and 

 enters an epidermal cell of the host B, and develops into 

 a mycelium which fills the tissues, and gives rise to another 

 special type of pustule, also usually rust-red in colour, 

 which is called the aecidium. The aecidium pustules contain 

 spores, known as aecidiospores, which are incapable of 

 infecting plant B, but infect plant A by sending their germ- 

 tubes through the stomata of the epidermis. 



The mycelium on plant B generally produces, in addition 

 to aecidia, other pustules, known as spermogonia (or pycnidia) , 

 which bear minute spore-like bodies, which are generally 



