XI 



THE HOMOSPOROUS LEPTOSPORANGIAT^ 



355 



lari^c initial, more or less triangular in form when seen in 

 profile, but with the point sometimes truncate. Transverse 

 sections show that it is really a four-sided pyramid. The 

 young segments are very large, and it is possible that these 

 may sometimes assume the role of initials. Owing to the 

 slowness and irregularity of cell division it is difficult to trace 

 the limits of the segments beyond the youngest ones. They 

 usually form a spiral, but cases were sometimes encountered 

 where the segments were apparently cut off in pairs from 

 opposite sides of the initial cell. The root-cap arises in part 

 from special segments cut off from the outer face of the apical 

 cell, but also in part from the outer cells of the lateral segments, 

 as in the Eusporangiatse. The separation of the tissue system 



Fig. 183. — Osimmda regalis (L.). A, Section of young sporophyll passing through three very 

 young sporangia ; B, longitudinal section of an older sporangium ; t, the tapetum, x 325 (after 

 Bower). 



follows much as in Botryclduni. The plerome cylinder is large 

 and oval in section, but with poorly-defined limits, and it is 

 not possible to state positively whether it owes its origin 

 exclusively to the innermost cells of the segments. The large 

 central tracheae, as in Adiantuni, are very early distinguishable. 

 O. Claytoniana agrees on the whole with O. cinnamomea, but 

 the divisions are much more regular, and it approaches nearer 

 the typical leptosporangiate type, both in the arrangement of 

 the young tissues and the structure of the fully -developed 

 vascular bundle, which closely resembles that of the Polypodi- 

 aceae, and differs from the investigated species of Osnninda and 

 Todea in the better development of the endodermis, and in 

 having the pericycle of but one or two layers. 



