ALGAE AND BRYOPHYTA 66 1 



origin falls in readily with antithetic theory, under which it would be held to 

 be a primitive, not an acquired condition. Moreover, it accords with the 

 relatively simple form and structure of the sporogonium when mature : 

 this simplicity has made the recognition of the part played by sterilisation 

 easier in the Bryophytes than it is in plants where continued embryogeny 

 leads to a more complex state. 



But the details of this primary embryogeny are carried out differently 

 in Mosses and in Liverworts : in the former, after the first division which 

 separates the hypobasal cell, apical growth appears in the epibasal hemis- 

 phere with regular segmentation of a two-sided initial cell : in the latter 

 the segmentation in the epibasal hemisphere is not localised apically, but 

 after division into octants the growth is intercalary. Both of these types, 

 so distinct in their details of segmentation, present points of interest for 

 comparison with the more complex embryogeny of Pteridophytes : but the 

 analogies offered by the Liverworts are the more instructive. In some of 

 them (Ricciaceae) there is no distinction of apex and base : it may be a 

 question whether this absence of polarity is primitive or acquired. In 

 others (Marchantiaceae) there is definite polarity, the whole hypobasal 

 hemisphere serving functionally as a foot and seta, while the epibasal is 

 reproductive. In others again (Jungermanniaceae) the hypobasal hemis- 

 phere develops into a unicellular appendage of small size ; the epibasal 

 hemisphere after octant division undergoes intercalary growth, with repeated 

 transverse segmentation : the seta is derived from the lower tiers of cells 

 thus produced, and it may be only the uppermost tier that remains fertile 

 (Figs. 124, 125). The interest here lies in the deferring of the propagative 

 function, as compared with the previous cases : the part which is in them 

 an absorptive seta is here a small body, with probably a minor or 

 temporary function, while the lower part of the epibasal region, which is 

 elsewhere propagative, takes up the duty of the hypobasal. The propa- 

 gative function is relegated to the apical tier, and thus, on a basis of com- 

 parison along the Liverworts, an example is established of that process of 

 deferring of the event of spore-production which is an essential feature in 

 the theory here put forward. A somewhat similar process has been traced 

 in the Mosses ; and in the Pteridophytes there is reason to believe that it 

 has been very prevalent. The presence of such evidence from phyla which 

 have probably been distinct from one another at least in the later phases 

 of descent, illustrates what is believed to have been a progressive 

 development which owes its prevalence to the fact that it was dictated 

 by biological advantage. 



The similarity of the small hypobasal appendage in the Jungermanniaceae 

 (Fig. 125) to the suspensor in certain Pteridophytes is a further point 

 for comparison ; but it is doubtful whether this is in reality anything more 

 than a very distant analogy. In either case the body in question represents 

 a part of the zygote which takes no active part in the further embryonic 

 development : both owe their origin to a form of meroblastic segmentation. 



