Dec. 23. 1918 Parasitism of Cronartiiim ribicola . 635 



UREDINIA 



The development of the uredinium is illustrated in Plate 51, A, B, and 

 55, A-C, which form a series from the very beginning of the formation 

 of the sorus to its maturity. When the uredinium starts to form, the 

 fungus cells may be found aggregated in groups in some large air space, 

 generally near a stoma. Certain of the cells in each group become 

 oriented with their long axis more or less at right angles to the epidermis, 

 against which they are closely appressed (PI. 55, A). These cells are 

 functionally equivalent to the basal cells of the mature sorus, in that 

 from them arise the cells which go to make up the peridium, and the first 

 urediniospores. The first division of any one of the vertically elongated 

 cells results in the formation of a cell which adheres to its neighboring 

 homologous cells to make the peridium (PI. 55, B, a). In the second 

 division the first urediniospore initial cell, or its equivalent, is cut off. 

 This divides immediately to form the first urediniospore and stalk cell 

 or their equivalents. The position of the cells referred to, as they appear 

 in a young uredinium, are illustrated in Plate 55, B, in which a is the 

 peridial cell, b the young urediniospore or its homolog, c the stalk cell 

 or its homolog, and d the basal cell. It will be noted that the arrange- 

 ment of the cells suggests that the sorus is made up of a compact aggre- 

 gation of vertical rows of cells. This arrangement is temporary for the 

 middle region of the sorus, but permanent for the circumference of the 

 sorus. Plate 55, B, which represents a median section from a young 

 uredinium, can be duplicated by taking sections through the edge of any 

 mature sorus. This fact should be kept clearly in mind in any discussion 

 of the structure of the uredinium. 



The first urediniospores mature in the middle of the sorus (Pi. 51, 

 B, 6). Their formation and method of growth corresponds closely with 

 the production of normal stalked urediniospores in other rusts. To re- 

 peat the process suggested above: The basal cell undergoes division 

 (PI. 55, C, c; 59, E-I). The upper cell is the urediniospore initial (Pi. 

 59, J, Iv) ; the lower cell is potentially the equivalent of the original basal 

 cell. The urediniospore initial now divides (Pi. 59, K, L, M) to form a 

 larger upper cell, the urediniospore, and a smaller lower cell, the stalk 

 cell (PI. 51, B, h, c; 55, C, d). While this process is going on, the layer 

 of cells constituting the peridium gradually separates from the underly- 

 ing urediniospores along the line between the cells marked "a" and 

 "6" in Plate 55, A. The figures in Plate 51, A and B, represent steps in 

 the process leading up to conditions shown in Plate 55, C. As the uredinio- 

 spores grow, the peridium (Pi. 51, A, a; B, a) is forced up into a dome. 

 The individual peridial cells lose the regular shape and outline shown in 

 Plate 59, B, a, and become irregularly compressed or obliquely flattened. 

 The growing urediniospores develop pressure against the epidennal cells, 

 which flattens them out and finally causes them to be torn apart (Cf. 

 PI. 51, A, B, with PI. 55, C). The break in the epidermis frequently 

 88090°— 19 2 



