16 MICEOBES, FERMENTS, AND MOULDS. 



of wheat, but only upon the leaves of another plant, 

 the barberry. 



Borne by the dew or by a drop of rain on to the 

 young leaves of the barberry, the teleutospores germi- 

 nate, and form reddish-brown patches which affect 

 both sides of the leaf. On its lower surface the 

 spores are smaller, and are termed spermata; their 

 function is not thoroughly understood. The larger 

 spores on the upper ' surface are called cecidiospores 

 (Fig. 6), and with these we are more concerned, since 



Fig. 6. Section of a barberry-leaf bearing two cecidiospores, more or less developed, 

 of Puccinia graminis (much magnified). 



they are destined to return to the wheat, rye, or other 

 grasses, in order to reproduce the original rust. 



When they are placed on a blade of one or 

 other of these grasses, the oecidiospores germinate at 

 once, and it is soon covered with patches resembling 

 those of the preceding year; when these patches are 

 numerous, they dry up the blade and destroy the ear. 



Hay and straw affected by rust should never be 

 given to animals as food, since such food may produce 

 disease. 



Thus it appears that Puccinia graminis presents 

 the phenomenon of alternation of generations ; that is, 



