DEVELOPMENT OF PLANTS 



223 



ess has apparently entirely disappeared, although Blackman has 

 called attention to indications of reproductive organs in the early 

 development of the cluster cups and also to the fact that there 

 is a fusion of nuclei in certain cells of the mycelium, similar to 

 the fusion of gametes before the aecidiospores are developed. 



The aecidiospores 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. However, this mycelium forms in place of cluster 

 cups, groups of erect hyphae at the ends of which single reddish 

 spores are formed (Fig. 160, C). This growth ruptures the 

 epidermis, exposing the spores in rusty lines or blotches (Fig. 

 1 60, 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 

 uredospores. This phase of the fungus is a very destructive 

 one/7or the spores are formed in great numbers and provided 

 with thin walls. They are widely distributed by the wind and 

 germinate at once (Fig. 161, A) on other wheat plants which 

 soon show the rusty brown streaks of uredospores. In this way, 

 the pest spreads with great rapidity from a single center of in- 

 fection. This formation of uredospores 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 uredospores but are provided with thick 

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

 end of the hyphae (Fig. 160, D). Consequently, when the epider- 

 mis is ruptured, these spores form rusty black blotches on the 

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

 season's growth. These spores, called teleutospores, are resting 

 spores and tide the fungus over the winter. They germinate 

 in the spring quite independent of any plant and being dependent 

 upon the food stored in the spore, they only form short hyphae 

 which usually become divided into four cells (Fig. 161, B). 

 This structure is known as the basidium. Each cell of the basi- 



