28o MOSSES AND FERNS chap. 



radiating from the nuclei and connecting adjacent ones (Fig. 

 146, C). The first traces of the division walls appear simul- 

 taneously between the nuclei in the form of cell plates composed 

 of minute granules, probably of cellulose, which quickly coalesce 

 and form a continuous membrane. In this way the upper 

 part of the spore becomes transformed into a solid tissue (Fig. 



147)- 



The cell formation proceeds quickly toward the base of the 



spore, following the spore wall, so that for a time the central 

 space remains undivided. The whole process recalls most 

 vividly the endospore formation of most Angiosperms. On 

 account of the extremely thin walls and dense contents of the 

 young prothallial cells it is not easy to determine exactly when 

 the whole spore cavity becomes filled up with cellular tissue. 

 Because of the greater number of free nuclei in the upper part 

 of the spore, and their consequent close proximity, the cells 

 are smaller than those in the central and basal parts of the 

 prothallium. Sometimes the transition from this small-celled 

 tissue to the large-celled tissue of the basal part is quite abrupt, 

 and the more noticeable as the upper cells are more trans- 

 parent ; but there was nothing to indicate that this was in any 

 way connected with the early divisions of the primary nucleus, 

 and more often no such sudden transition was seen. 



Hofmeister's account of the coalescence of previously 

 separate cells to form the prothallium was obviously based 

 upon incorrect observation, and is not borne out by a study of 

 sections of the germinating spore. 



The first archegonium is very early evident, generally 

 before the cell division is complete in the lower part of the 

 spore. It occupies the apex of the prothallium, and the 

 mother cell is distinguished by its large size and dense granular 

 contents. It is simply one of the first-formed cells that soon 

 ceases to divide, and as its neighbours divide rapidly the 

 contrast between them becomes very marked. Whether seen 

 from above or in longitudinal section, it generally is triangular, 

 or nearly so. In the structure of the mature archegonium, 

 OpJdoglossum, to judge from Mattenius' somewhat incomplete 

 account, shows strong points of resemblance, as do the 

 Marattiaceae. 



The development of the archegonium corresponds almost 

 exactly with that of Maraitia, but the basal cell is always 



