DEVELOPMENT OF PLANTS 235 



as will be seen from the outline given below, is so different from 

 that of the algae and is known in so few^ forms that any interpre- 

 tation of the phenomena should be deferred for the present. 

 The spore-producing hyphae in the young aecidia form a rather 

 loose strata of somewhat elongated cells (Fig. 159^, i). These 

 cells divide, forming an upper series of cells which are smaller 

 and narrower than the lower or basal cells. The upper cells are 

 sterile, ultimately disappearing, and they have been compared 

 to remnants of trichogynes. 



The basal cells enlarge, become inclined towards one another 

 in pairs and finally meet at their upper ends, the walls dissolving 

 at the point of contact. The nuclei of the two cells now pass 

 to the fused region of the cells and divide, forming four nuclei. 

 Two of these wander back into the base of their respective cells, 

 while the other two pass to the upper portion of the fused area 

 and become separated from the basal nuclei by a transverse 

 wall (Fig. 159^, 5). The upper cell formed in this manner is 

 the spore mother cell and it divides at once, forming an aeciospore 

 and a small intercalary cell. As soon as this process is completed 

 the tw^o nuclei at the base of the fused cells move up again into 

 the fused portion of the cells, which region has elongated in the 

 meantime, and divided as before. In this way a chain of aecio- 

 spores is formed, separated by small intercalary cells (Fig. 159^, 

 6) that soon disintegrate. 



As soon as the cluster cups are ruptured the aeciospores are 

 scattered by the wind, and singularly, will only germinate and 

 infest the seedlings and young leaves of the wheat in which they 

 develop a mycelium similar to that of the barberry. However, 

 this mycelium forms in place of cluster cups, groups of erect 

 hyphae at the ends of which single reddish spores are formed 

 (Fig. 160, C). This growth ruptures the epidermis, exposing 

 the spores in rusty lines of blotches (Fig. 160, B). These yellow- 

 brown blotches account for the popular name of Rusts given to 

 this group of fungi. This is the summer or uredinal stage of the 

 parasite. The spores are known as urediniospores. This phase 

 of the fungus is a very destructive one, for the spores are formed 

 in great numbers and provided with thin walls. They are widely 



