388 



THE NON-VASCULAR PLANTS 



Late in summer another kind of spore appears. This is 

 usually after the wheat has been cut. We find this new 



kind of spore on the stubble. 

 Stubble is what is left of the 

 wheat after the harvest. This 

 second kind of spore forms black 

 marks on the old stalks; it is 

 sometimes called black rust. 

 These are the winter spores of 

 the wheat rust. These, black 

 spores have heavier walls than 

 the summer spores, and it is by 

 means of them that this parasite 

 lives over winter. 



The winter spores germinate 

 in early spring and a third kind 

 of spore appears as a result of 

 their germination. These winter 

 spores are well stocked with food 

 and the generation which they pro- 

 duce is a little filamentous structure 

 which is not a parasite at all; its 

 nourishment is drawn from the sup- 

 ply stored in the spore. (See Figure 

 i88.) This third kind of spore is 

 borne at the ends of very slender 

 branches of the filaments. The fila- 

 ment from which these branches 

 arise is a basidium, the structure 

 you have already noted in connec- 

 tion with toadstools. This kind of 

 spore is called the early spring spore. 



Fig. 185. — A leaf of lilac showing 

 the mildew {Microsphara) upon 

 it. The dark dots are ascocarps. 

 They are explained in the context. 



Fig. 1 86. — An ascocarp of the 

 lilac mildew. Note the 

 peculiar forked outgrowths. 

 The ascocarp has been 

 crushed, and two asci with 

 their contained spores may 

 be seen protruding at the 

 right. 



