PLATE 55 

 Cronartium ribicola: 



A. — A drawing of a median section through a very young uredinium. Note that 

 the fungus cells are closely compacted against the underside of the host epidermal 

 cells, and that the binucleate cells adjacent to the epidermal cells are oriented with 

 their long axis more or less perpendicular to the epidermis. The section from which 

 the drawing was made was cut just to one side of a stoma. See text for further expla- 

 nation. X 500. 



B. — A drawing of a median section through a yoimg uredinium, somewhat older 

 than that illustrated in figure A. a, A peridial cell ; b, a young urediniospore ; c, a cell 

 which is homologous to the stalk cell of an older sorus; d, a basal cell. See text for 

 further explanation. X 500. 



C. — A drawing of a median section through a small mature uredinium. a, A binu- 

 cleate basal cell; b, a tetranucleate basal cell just before the urediniospore initial is 

 cut off; c, a secondary lu-ediniospore initial which has just been cut off from the basal 

 cell ; d, the iu"ediniospore initial has divided into a urediniospore and a stalk cell ; e, 

 a matiu^e urediniospore which is still connected with the basal cell by a collapsed stalk 

 cell ; /, crushed epidermal cells of the host; g, the bank of parenchyma-like cells which 

 encircle the sorus. The cells of this group, which lie next to the epidermis, are homo- 

 logous to the thickened peridial cells which overlie the greater part of the uredinium. 

 Note the comparatively small opening in the peridium. The drawing has been slightly 

 diagrammatized for the sake of completeness by combining some of the features from 

 two sections. Compare Plate 51, B. X 500. 

 88096°— 19 4 



