190 



Mycologia 



subterminal cells and form the teleutospores. Some of the young 

 buds coming against the cork cells are bent sidewise or even 

 downward because the cork layer has not as yet been sufficiently 

 raised (Fig. 7). This figure shows three buds in which the two 

 nuclei are dividing. As Blackman has published good figures of 

 the later stages, no further study was made of spore formation. 



Gymnosporangium nidus-avis 



The three stem forms, G. nidus-avis, G. clavariaeform e, and 

 G. clavipes sometimes develop sori on the blades of leaves. A 

 comparison of such leaf sori in the -three species discloses in- 

 teresting differences in the way in which the primordium de- 

 velops. The first two species develop mycelium in all parts of the 

 leaf, the hyphae ramifying among the wood cells. G. clavariae- 

 forme causes a considerable enlargement or hyperplasia, G. nidus- 

 avis induces some enlargement of individual cells of the leaf, but 

 the total effect is not very striking. G. clavipes forms a sorus 

 which is very superficial and the mycelium is very much localized. 



The material .that was used in the study of G. nidus-avis was 

 taken from artificially infected plants, nos. 929 and 609, and from 

 naturally infected plants where the sori develop on rather con- 

 spicuous gall-like mounds beneath the cork. The sorus is formed 

 in much the same way as we found in G. clavariaeforme. The 

 cork may be somewhat thicker in the specimens examined and 

 the pseudoparenchyma even more extensively developed and com- 

 pact. Figure 8, PI. 10, shows the marginal region of a sorus, the 

 main part of which had already developed mature spores. The 

 terminal cells begin to degenerate before they elongate very much. 

 Their nuclei are often smaller than are those of the basal cells at 

 this stage. All cells of the primordium are more or less rectan- 

 gular and are closely crowded together. At the right in the figure 

 one basal cell has budded out laterally and another has pushed 

 into the buffer cell. Other stages are similar to those described 

 in the preceding species. 



Conclusions 



The origin of the teleutospores from the subterminal cells of 

 the tissue composing the primordium has been shown to be a very 



