THE GAMETOPHYTE 



121 



Basal segments are cut off by a wall extending the whole depth of the prothallium 

 and the segment thus cut off is divided at once by a horizontal wall into a dorsal and 

 ventral cell of nearly equal size (fig. 88, E, d, v). Cell divisions are more active in the 

 ventral segments, more manifestly so at some distance back from the apex. It is due 

 to this more active cell division in the ventral segments that the strongly projecting 

 cushion-like mass of tissue is formed upon the ventral surface of the prothallium, 

 upon which the archegonia later make their appearance. The superficial cells of 

 both surfaces of the prothallium have a thick cuticle which often makes it difficult 

 to embed the p roth alii a without bad shrinkage. From the under side of the prothal- 

 lium numerous rhizoids are developed, which in the case of Marattia and Atigiopteris 

 are unicellular and thin-walled, but in Dancea become divided into several cells. 

 Sometimes there seems to be no definite apical growth in the early stages, but on the 

 other hand Jonkmann states that both of the superior cells may function as apical 

 cells, thus inaugurating the early dichotomy of the young prothallium, and even a 

 third branch may arise from one of the inferior quadrants, which assumes the 

 character of a third apical cell. 



Among the leptosporangiate ferns, the forms which most nearly resemble the 

 Marattiacea? in the development of the gametophyte are the Osmundaceae, especially 



Fig. SS. 

 A-C. Threr young gametophytes "t Atigiopteris. </\ spore membrane. A, x'ioo; B, C, .160. (After fonkmann.) 



I), horizontal, E, longitudinal, ections of the prothall 



apex oi Marattia douglasii. ,v *, initial cells. 



the genus Osmunda (Campbell, Mosses and Ferns, 2d ed., pp. 347, 348). In 

 Osmunda there is often a tendency also to the formation of a massive structure at an 

 early stage in the development, due to the formation of cell divisions in three planes, 

 so that the prothallium becomes from the first more than one cell thick. This 

 tendency in the Marattiacea' becomes much more pronounced as the gametophyte 

 grows and very soon there is evident a veiy thick midrib, which often becomes 

 exceedingly conspicuous in the older gametophyte, unlike the typical ferns, where 

 the thickening of the prothallium is confined to the region which bears the arche- 

 gonia. In the Marattiacea? this thickening extends almost to the margin of the 

 prothallium, so that it is only at the extreme edge that the prothallium shows but a 

 single layer of cells. The very old prothallia in Marattia branch dichotomouslv 

 (fig. 87, E, E), and the process is entirely similar to that found in many thallose 

 liverworts. The original growing point becomes divided by the development of a 

 median lobe, thus inaugurating two growing points, and the midrib back of the 

 growing point forks in exactly the same way as in many liverworts. 



Besides this dichotomous branching, it is not at all uncommon to have adven- 

 titious buds developed upon the margin of the thallus. These form small secondary 



