RUSTS 19 



and stems, the red rust. The urediospores can also infect only the 

 wheat, not the barberry, and they spread the disease rapidly. 



Toward the end of the summer the host plant matures and the 

 infection tends to develop more abundantly on the stems and from 

 the mycelium on the stems there arises a fifth type of spore, the 

 teliospore (teleutospore). The teliospores are surrounded by a thick 

 dark-colored cell wall and are formed in the black pustules on the 

 stems, the black rust. They are two-celled, each spore consisting of 

 two separate protoplasts, each in its own compartment (Fig. 10). 

 Each cell is at first binucleate but, 

 in the teliospore, the nuclei fuse 

 and the uninucleate, diploid stage is 

 found. 



The teliospores remain dormant 

 over winter. In the process of ger- 

 mination the fusion nucleus under- ^^"- ^\ Germination of a telio- 

 , T • • .1 spore oi ruccinia graminis showing 



goes reduction division as m the the formation of promycelium and 

 smut spores. The four nuclei be- basidiospores. 



come separated by crosswalls in 



the promycelium so that we have a four-celled basidium as in some 

 of the smuts (Fig. 11). From each of these four cells a sporidium, 

 which may be considered a basidiospore, is formed, each with but 

 one haploid nucleus. These spores are capable of infecting the bar- 

 berry but not the wheat. 



This complex life cycle on two hosts is not a special case but is 

 typical of many of the rusts. Resemblances to the cycle of the mush- 

 rooms and smuts can be seen. The complete cycle is known for a 

 large number of rusts including many of great economic importance. 

 Sometimes the greater economic loss is in the alternate host, that is, 

 the host which corresponds to the barberry in the disease just con- 

 sidered, and in which cell union but not nuclear fusion takes place. 

 Many of the species of rusts complete their life cycle on a single 

 host, and are said to be autoecious. Many, however, like P. graminis 

 do require two different hosts. These are heteroecious. The two 

 host plants required for heteroecious rusts are themselves not closely 

 related, as for instance wheat and barberry, the hosts of the stem 

 rust of wheat, currants and white pine, the hosts of the white pine 

 blister rust, cedars and apple trees, the hosts of the cedar rust of 

 apples. It might be of interest to bacteriologists that the necessity 

 of two different hosts for P. graminis to complete the sexual cycle 

 and the various stages in the life cycle were known in essential de- 

 tails by 1865, largely as a result of the work of the Tulasne brothers 



