OF ARTS AND SCIENCES. 207 



The uredospores have been observed only on the leaflets ; the 

 heads, on the other hand, have been seen not only in this position, but 

 also on the leaf rhachis and the young portions of the stem proper, 

 in both of which positions the clusters are denser than on the leaflets. 

 Whether the heads on the stem and rhachis do or do not originate 

 in cavities previously occupied by the uredospores cannot here be 

 definitely stated. 



For the more careful study of the individual heads the following 

 methods have been found of advantage. For thin sections, portions 

 of the leaflet bearing the fungus were embedded in paraffine, and 

 sections were cut with a Jung microtome as employed in animal 

 histology. Material to be softened or cleared either for hand sections 

 or teasing sliould be boiled in a strong solution of potassic hydrate 

 until the bi-owii color of the heads noticeably tinges the reagent. Fair 

 results in maceration were attained when this process of boiling was 

 continued beyond the point above mentioned; but care must be exer- 

 cised lest the boiling should go so far as to discliarge completely the 

 color from the heads. In this condition tiie parts are too transparent, 

 and their separation not so easily accomplished as at that stage where 

 some color remains. 



In outline, the head (Fig. 2) appears as a swollen, umbrella-like 

 mass, rounded or 'flattened above and connected with the host by a 

 moderate stalk. Three regions may be defined in it: first, the spore- 

 mass or brown cap-like cluster of cells at the top ; second, the cyst 

 region, composed of cells with thin transparent walls connecting the 

 spore-mass with the third or stalk region, consisting of a series of 

 compressed parallel cells passing from the cysts to the leaf tissue 

 below. 



The spore-mass is a low dome of cells whose outlines by mutual 

 pressure have become irregularly polygonal. The number of cells as 

 viewed from above varies from two or three in extreme cases (Fig. 5), 

 to as many as fifty, the average being perhaps thirty (Figs. 3 and 4). 

 Externally these cells may be divided into two groups, the marginal 

 cells or those on the periphery of the dome (Fig. 3 a), and the central 

 cells, including all those which do not come to the edge (Fig. 3 b). 



A cross-section of the spore-mass is a low arch of cells (Fig. 6) in 

 which the marginal cell a is seen to occupy a position which in the 

 central region is taken up by two cells, one external, b, and the other 

 internal, c. This arrangement, although generally constant for most 

 heads, nevertheless has its exceptions ; for, in some instances, the 

 position of a marginal cell is occupied by two cells bearing the same 



