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A TEXT-BOOK OP BOTANY 



FIG. 141. The winter 

 spores of wheat-rust. 



means of these summer spores the rust may spread through 

 a field of wheat and into adjoining fields with great 

 rapidity. 



Later in the season, on the stubble 

 and on plants not removed in the har- 

 vesting, black lines and dots appear, 

 which are masses of a very different kind 

 of spore sent to the 

 surface by the myce- 

 lium (Fig. 141). This 

 spore, which is two- 

 celled and has a very 

 heavy wall, is the 

 winter spore; for it is 

 in this form that the 

 rust usually endures the winter. 



In the spring the winter spores, lying 

 where the plants on which they were 

 produced have decayed, begin to ger- 

 minate, each one of the two cells send- 

 ing out a short filament. This filament 

 is not a parasite, but a saprophyte, and 

 usually consists of four cells, each one 

 of which sends out a little branch, at 

 the tip of which a small spore is pro- 

 duced (Fig. 142). These may be called 

 early spring spores. 



These early spring spores are scat- 

 tered by the wind; and those falling 

 upon barberry leaves germinate, the 

 new mycelia entering and spreading 

 through the leaves. In this phase the 

 rust is parasitic upon an entirely dif- 

 ferent host, and one that holds no relation to wheat. The 

 mycelium in the barberry leaves sends to the leaf surface, 



FIG. 142. A winter 

 spore of wheat - rust 

 germinating, each fila- 

 ment producing four 

 cells, each of which 

 sends out a branch 

 that produces at its tip 

 a spore (early spring 

 spore). After Tu- 



LASNE. 



