66 TEXTBOOK OF BOTANY 



appear mixed with summer spores in the same sorus (Fig. 28). 

 But in the sori that are formed later, only winter spores are 

 produced. Each winter spore has a slender stalk and con- 

 sists of two cells whose walls are thicker and darker than 

 those of the spring and summer spores. A sorus containing 

 only winter spores is black, and so the appearance of these 

 sori is spoken of as the " black rust." Thus the " red rust " 

 and the " black rust " are different stages in the history of 

 the same disease. 



Each cell of a young winter spore contains two nuclei (as does the 

 summer spore) ; but as the spore ripens, the two nuclei in each cell 

 unite, so that the mature spore consists of two one-nucleate cells. 

 Since the two nuclei which are present at first in each cell of the winter 

 spore are direct descendants of the two nuclei of the spring spore, the 

 union of nuclei in the winter spore is really the completion of the sexual 

 process which began with the union of cells at the base of the cluster 

 cup just before the formation of spring spores. Whereas in Spirogyra 

 nuclear union follows closely upon the cell union which produces .the 

 zygote, cell union and nuclear union in the rust are separated by a 

 whole generation of the rust plant that generation, namely, which 

 lives in the tissues of the wheat. 



95. Germination of Winter Spores; Sporidia (Fig. 29, E). 

 The winter spores are adapted by their thick walls and 

 probably by the comparatively dry condition of their pro- 

 toplasm to endure unfavorable conditions. They represent 

 a stage, therefore, in which the rust may live through the 

 winter. They remain upon the ground or the stubble, or are 

 scattered widely in the harvesting and threshing of the grain. 

 In order to produce new plants, they do not require, as the 

 spring and summer spores do, a particular host upon which to 

 grow. On the contrary, they germinate wherever they may 

 be when warm weather returns in the spring, provided 

 sufficient moisture is present. Each cell of the winter spore 

 can put out a germ tube and develop a new plant, so that the 

 " winter spore " is practically two spores that have remained 

 together. The plant that grows from each cell of the winter 



