210 PROCEEDINGS OF THE AMERICAN ACADEMY 



hypha be marginal, is unilocular; if central, bilocular. If the mor- 

 phology of the Ravenelia head be such as we have indicated, it is but 

 natural to expect that confirmatory evidence should be found in its 

 young stages. 



Tlie young heads first make their appearance amongst the para- 

 physes which line the cavities previously occupied by the uredospores. 

 Portions of the leaflet on which these cavities were quite numerous 

 were boiled in a weak solution of potassic hydrate, and picked to 

 pieces with needles. Material thus teased contained the young heads, 

 sometimes isolated, and sometimes attached to small fragments of leaf 

 tissue. 



The youngest head yet found (Fig. 12) consisted, in optical section, 

 of four hyphaj, each of which is divided by cross partitions into three 

 regions corresponding probably to the spore, the cyst-cell, and the 

 stalk-cell of the matured head. Younger stages than this were diffi- 

 cult to distinguish from mere clusters of paraphyses, which are of not 

 unfrequent occurrence. The individual paraphyses are generally 

 smaller than a single hypha of such a specimen as that seen in Fig. 

 12, yet some of them attain a size much exceeding this, and interme- 

 diate clusters could probably be distinguished from very young heads 

 only by the fact that their individual filaments would separate readily, 

 whereas those of a true head are quite firmly bound together. It is 

 probable that the hypha cluster destined to form a head rises from the 

 mycehum at the base of the paraphyses, and from its first appearance 

 is more or less consolidated. The stage seen in Fig. 12 is a slight 

 advance on this condition. The head has begun to rise, and the 

 hyphfE are elongating ; the two primary sets of cross partitions have 

 appeared, being the first intimations of the spore, cyst, and stalk re- 

 gions. All the cells are filled with a granular protoplasm. A still 

 more advanced stage is seen in Fig. 13. This head was almost com- 

 pletely isolated from the surrounding leaf tissue and mycelium, and 

 consequently shows more of the stalk region than is seen in Fig. 12. 

 The spore-mass has become proportionally larger than in the earlier 

 stage, and a plane of separation dividing its central cells into an ex- 

 ternal and an internal series has been developed ; the increasing thick- 

 ness of the walls of the spore-cells is noticeable, but no brown color 

 has as yet appeared. The cells destined to become the cyst-cells are 

 still unmodified. The stalk-cells are slightly elongated. Below these 

 last, and similar to them in appearance, is another set of cells, of 

 which no trace has been found in the matured head. In the later 

 stages these doubtless serve as a basement structure, usually hidden 



