52 



ELEMENTARY STUDIES IN BOTANY 



looking lines on the stem and leaves of some of the plants. 

 These lines extend and multiply, new plants become in- 

 fected, and presently the whole field may become rusty. A 

 microscope shows that this rusty looking material is made 

 up of spores (Fig. 35) ; and it is evident 

 that they have been brought to the surface 

 by sporophores arising from an internal 

 mycelium. 



Later in the season, after the wheat 

 has been harvested, there appear black 

 lines on the stubble, the so-called " black 

 rust." It does not belong to the stubble 

 any more than to the rest of the plant, but 

 it appears so late in the season that ordi- 

 narily there is only stubble left to appear 



FIG. 37. Winter 

 spores of the wheat 

 rust germinating, 

 each filament con- 

 sisting of four cells, 

 and each cell send- 

 ing out a delicate 

 branch that pro- 

 duces at its tip 

 a spore (early 

 spring spore). 

 After TULASNE. 



FIG. 38. A cluster-cup (on barberry) of the wheat rust, 

 containing rows of spring spores ; each spore contains 

 two nuclei. 



upon. The " black rust " consists of heavy- walled spores that 

 arise from the same mycelium (Fig. 36). There are thus 

 two kinds of spores produced by the mycelium on the 

 wheat ; one kind during the season, by means of which the 

 rust is spread ; the other kind towards the end of the season. 



