X THE HOMOSFOROUS LEPTOSPORANGIAT^ 349 



As soon as the apical cell is established, growth proceeds 

 as in Onoclca, and a heart-shaped prothallium is formed. One 

 difference, however, may be noted. Each segment cut off from 

 the apical cell divides first by a transverse wall into an inner 

 and an outer cell, but the inner cell from the first undergoes 

 divisions by horizontal walls, so that a central midrib is formed, 

 very much as in Met::geria, and the prothallium becomes more 

 elongated than is common in the PolypodiacCce. The single 

 two-sided apical cell persists for a long time, but is finally 

 replaced either by a single cell, much like that of PcUia 

 cpiphylla, or more commonly by a series of marginal cells, as 

 in the Marattiaceae or Polypodiaceas. The subsequent growth 

 of the prothallium is the same as in those forms, but no definite 

 relation could be made out between the archegonia and the 

 segments of the initial cells. Among the Hepaticae Dendro- 

 ceros offers almost an exact analogy in the form of the apical 

 cells and the divisions of the segments. 



According to Luerssen (3), in Todea a distinct apical cell 

 is often wanting, and the growth throughout is due to the 

 activity of several similar initials. His figures, however, 

 hardly bear out his statement, and further information is de- 

 sirable on this point. 



As the prothallia grow older the midrib becomes conspicu- 

 ous, and projects strongly from the ventral surface. In O. 

 cinnarnomca and O. rcgalis even at maturity it is very little 

 broader where the archegonia are formed; but in 0. Claytoni- 

 ana it forms a cushion in front, much like that of Marattia or 

 the Polypodiacese, and in this respect, as wd\ as in the form of 

 the apical cells, seems to approach the latter. In this species 

 the prothallium is lighter coloured, and the rhizoids not so 

 dark, while in its dark green colour and fleshy texture 0. ci}i- 

 iiamornea recalls Anthoccros Iccvis or Marattia. 



Where a cell mass is formed at first, this condition is tem- 

 porary, and an apical cell is established which gives rise to the 

 ordinary flat prothallium. The small male prothallia, which are 

 produced in large numbers, exhibit various irregularities and 

 quite commonly do not show any definite apical growtli, and in 

 O. Claytoniana especially often branch irregularly, or in some 

 cases tiiere is a true dichotomy (Fig. 193, A.) Slender fila- 

 mentous prothallia are especially common in this species (Fig. 

 194, C), and recall somewhat those of some species of Trich- 

 omanes. 



