254 FORMATION OF SPORES IN THE RUSTS 



and become separated from the basal nuclei by a transverse 

 wall (Fig. 162, A, 3-5). The upper cell formed in this manner is 

 the spore mother cell and it divides at once, forming an aeciospore 

 and a small intercalary cell. As soon as this process is completed 

 the two nuclei at the base of the fused cells move up again into 

 the fused portion of the cells, which region has elongated in the 

 meantime, and divided as before. In this way a chain of binu- 

 cleate aeciospores is formed, separated by small intercalary cells 

 (Fig. 162, A, 6) that soon disintegrate. 



As soon as the cluster cups are ruptured the aeciospores are 

 scattered by the wind, and singularly, will only germinate and 

 infest the seedlings and young leaves of the wheat in which they 

 develop a mycelium similar to that of the barberry, but note that 

 the cells of this mycelium are binucleate like the aeciospores. 

 However, this mycelium forms in place of cluster cups, groups of 

 erect hyphae at the ends of which single reddish spores are formed 

 (Fig. 163, C). This growth ruptures the epidermis, exposing 

 the spores in rusty lines of blotches (Fig. 163, B). These yellow- 

 brown blotches account for the popular name of Rusts given to 

 this group of fungi. This is the summer or uredinal stage of the 

 parasite. The spores are known as urediniospores. This phase 

 of the fungus is a very destructive one, for the spores are formed 

 in great numbers and provided with thin walls. They are widely 

 distributed by the wind and germinate at once (Fig. 164, A) 

 on other wheat plants which soon show the rusty brown streaks 

 of urediniospores. In this way, the pest spreads with great rap- 

 idity from a single center of infection. This formation of ure- 

 diniospores goes on during the summer, sapping the vitality of 

 the plant and, in severe cases of infection, materially interferes 

 with the maturing of the grain. 



Later in the season, this same mycelium forms in the leaves 

 of the wheat quite a different type of spore. They are formed in 

 the same manner as the urediniospores but are provided with 

 thick dark walls and from one to several spores are developed at 

 the end of the hyphae (Fig. 163, D). Consequently, when the 

 epidermis is ruptured, these spores form rusty black blotches on 

 the leaves. This third stage is known as the telial, since it ends 



