JUNGERMANNIALES 



265 



the basis of the first segmentations, of the capsule from the seta. So far 

 as segmentations offer a basis for argument, the general conclusion may 

 be drawn that the seta and capsule are not always distinct ab ijntio. The 

 developmental facts suggest that the fertile region may be held to be a 

 residuum left by sterilisation, which has been basifugally progressive : the 

 result of such sterilisation is the region of the seta as it is seen in these 

 plants. We may regard as the most primi- 

 tive case that in which all the tiers of cells 

 of the embryo form the archesporium : 

 those cases in which the differentiation 

 of the archesporium is deferred in the 

 individual life may be held to be relatively 

 less primitive. 



The four cells of the uppermost tier, 

 which thus as a rule form the capsule 

 in the Jungermanniales, divide first 

 transversely to form four terminal cover- 

 cells : the four larger cells below again 

 segment to form four inner cells and 

 eight to twelve peripheral cells. This 

 is the same segmentation as occurs also 

 in the lower tiers : the similarity is in 

 accordance with the view above expressed 

 as to the origin of the seta by basifugal 

 sterilisation, and supports the conclusion 

 that seta and capsule had a common 

 origin. 



However interesting such questions 

 may be, they are more or less speculative. 

 A much greater interest, proportional to 

 the greater cogency of the facts, attaches 

 to the various modes of development of 

 the capsule itself in the Jungermanniales. 

 Jt has been seen that the inner cells 

 above described constitute the arche- 

 sporium. In many of the Akrogynous 



Jungermanniaceae the cells, after repeated divisions, undergo a differen- 

 tiation as in the Marchantiaceae, into spore-mother-cells and sterile elaters : 

 these are associated in various ways, and the case of Porella will serve 

 as an average example (Fig. 126). The argument from differentiation of 

 sterile and fertile cells during development applies equally here as in the 

 Marchantiaceae. The same is the case with many of the Anakrogynae; 

 but in some of the latter there is a more specialised tissue-differentiation 

 leading to the formation of a coherent mass of sterile tissue, with a more 

 definitely localised residuum of fertile tissue : this sterile mass has been 



FIG. 126. 



Porella Bolanderi. Longitudinal section of 

 a sporogonium after the final division of the 

 archesporial cells. ><35- (After Campbell.) 



