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TEXTBOOK OF BOTANY 



million bushels. The life history of this rust illustrates so well how diffi- 

 cult may be the problems encountered in attempting to control the fungi 

 that cause diseases of economic plants, that it will be described in more 

 detail than would otherwise be justified in a book on general botany 

 (Fig. 250). 



Fig. 250. Photomicrographs of the reproductive structures of stem rust (Pucciniu 

 graminis) of wheat: A, cross section of a leaf of common barberry with aecia 

 containing aeciospores (b) on the lower side and pycnia (a) on the upper; B, 

 a pustule of red summer spores (uredospores) that has broken through the 

 epidermis of a wheat stem; C, a pustule of teliospores in a wheat stem a little 

 later in the season; D, the following spring, a basidium bearing 4 basidiospores 

 has developed from each of the two cells of a teliospore. Hyphae from the basidio- 

 spores infect leaves of the common barberry and the cycle is repeated. 



During the summer the fungus is first apparent on the surface of 

 stems and leaf sheaths as patches of innumerable red spores (uredo- 

 spores). These spores are blown about by the wind and the hvphae from 

 them infect other wheat plants. New mycelia develop, and the spores 

 from them may be blown to other wheat plants. This process may be 

 repeated every week or ten days during the spring, thus spreading the 

 fungus rapidly over large areas. 



These same internal mycelia a little later produce many two-celled, 

 thick-walled black spores {teliospores) . These second spores in the life 

 cycle begin to appear before the wheat plant is mature, and they live 



