16 



STRUCTURE AND CLASSIFICATION OF FUNGI 



tensive proliferation by conjugate nuclear division the pairs of nuclei, 

 each pair in a separate cell, unite to form the only diploid cell in 

 the life history. After reduction division in this cell (in both), the 

 haploid uninucleate basidiospores are set free. These basidiospores 

 are of different mating types (two to four to a basidium). The close 

 relationships of the corn smut and the mushroom, so very different 

 in gross appearance, is clear from their cytological life histories. 

 Moreover in certain smuts the sporidia are discharged forcibly in 

 the same manner as the basidiospores of the mushrooms. 



Rusts. In the rusts (Uredinales) the life cycle is still more com- 

 plicated. The fungus {Puccinia graminis) causing black stem rust 

 of wheat may be taken as an example. It requires two hosts, the 

 common barberry and wheat, for its complete cycle. 



When a sporidium (or basidiospore), which is uninucleate, lodges 

 on a barberry leaf, it may give rise to haploid mycelium. Such my- 



FiG. 7. Section through a portion of a pustule on the upper surface of a bar- 

 berry leaf infected with Puccinia graminis showing pycnium, pycniospores (a), 

 and receptive hyphae (b). Redrawn from figure of A. -H. R. Buller, Nature, 



141, 33 (1938). 



