10 Robinson, Puccinia nialvaccaruin and its host plant. 



older and younger petioles showed that the fungus does 

 not merely prevent the formation of the thickening but 

 actually causes its disappearance. 



In addition to the enlargement of the cells of the 

 collenchyma, already referred to, there is a very distinct 

 increase in size of the cells of the assimilating tissue and 

 epidermal cells overlying the young sorus ( Text-Jig. yh), 

 as compared with the corresponding cells from an un- 

 affected part of the same petiole {Text-jig. /B). These 

 enlarged cells are entered by haustoria of the fungus. 



coii: 



■m^^<y^ 



Text-fisi. 7A. Epidermal and assimilating cells overlying the 

 margin of the teleutospore sorus. Mycelium 

 between, and haustoria wiihin most of these cells. 



X 200. 



B. Corresponding normal epidermal and assimilating 

 layers of petiole. x 200. 



All the evidence accumulated points to the probability 

 that the infection by the germ tube of a single sporidium 

 is capable of producing an area of infection such as has 

 been described. Numerous artificial infections have been 

 made, and the course of development of the fungus from 

 infection onwards has been studied both in seedlings and 

 mature leaves. Kellerman' states that a sporidial germ 



= Sitz. Ber. Phys. Aledic. Soc. Erlangeu, 1874. 



