258 



ESSENTIALS OF BOTANY 



(b) The rough outer coating and the inner coating with (usually) 

 four pits or thin places in its wall about midway of the spore. 



333. Black Rust, Minute Structure. — Re- 

 move a small mass of the black-rust spores. 

 (ieleutospores), mount, and examine as de- 

 scribed in Sect. 332. Note: 



(a) The shape and attachment of the spores 

 (Fig. 182). 



(J) The structure of the spore (composed 

 of two cells) and the thickness of the cell- 

 wall. Look for thin places or germinating 

 pores in the cell. 



334. Life History of Wheat Rust. — 



This species of rust under favorable cir- 

 cumstances shifts from the barberry as 

 a host>-plant to wheat or otlier grass and 

 back to the barberry again year after 

 year. The cluster-cup spores are borne 

 by the winds to grain or grass fields, 

 where they germinate and develop 

 red-rust streaks full of uredospores. 

 These spores are carried to other wheat 

 plants on which they germinate, and 

 so the growth of the fungus spreads. 

 In late summer or autumn the same 

 mycelium which earlier produced ure- 

 dospores forms only teleutospores and 

 cells which bear basid- the red-rust streaks turn blackish. The 



iospores, 6. These are , , , . ,, . , , 



carried hy the- wind teleutospores Survive the wmter and 

 to the barberry host- in the Spring germinate on barberry 

 leaves (Fig. 183), produce a new crop 

 of cluster-cup spores, and so on. Note that all these proc- 

 esses of spore formation are asexual; no gametes are known. 



Fig. 183. A Germinat- 

 ing Teleutospore of 

 Puccinia graminis. 



a, a' , threads produced 

 by the germination, 

 from the ends of which 

 grow short rows of 



