ANDREAEALES 



275 



progressive from within, which has 

 extended in Sphagnum^ as in Anthoceros, 

 to the whole product of the endothecium, 

 while in both the archesporium takes its 

 origin wholly from the amphithecium. A 

 key to this difference of Sphagnum from 

 all other Mosses may be found in Noto- 

 thylas, in which the sporogenous cells 

 may arise from both sources, the centri- 

 fugal progress of sterilisation being less 

 completely carried out there than in 

 Anthoceros. In both cases the difference 

 appears referable to the degree of centri- 

 fugal sterilisation in a body in which the 

 spore-production was originally central. 

 Thus the condition of Sphagnum is in 

 this respect the most advanced in the 

 Mosses, as that of Anthoceros is among 

 the Liverworts. As regards decentrali- 

 sation of the fertile tissue the rest of the 

 Mosses will be seen to correspond to 

 the less extreme types of the Hepatics, 

 while the columella-less sporogonia of 

 Notothylas link together the two degrees 

 of decentralisation. 



B. ANDREAEALES. 



The mature sporogonium of Andreaea 

 offers analogies with that of Sphagnum 

 in its form, with its short seta and large 

 foot, the whole being borne up on an 

 elongated pseudopodium : it also corre- 

 sponds in the fact that the columella is 

 interrupted at the apex^ and covered by 

 the archesporium which forms a complete 

 dome (Fig. 133); but it differs in the 

 dehiscence when mature by longitudinal 

 slits. 



The segmentation of the zygote corre- 

 sponds to that of other Mosses rather 

 than to that of the Hepatics, for after 

 the appearance of the transverse basal 

 wall, the cleavages are oblique, a two-sided initial cell being present (Fig. 

 134 A-D) : but the number of such segmentations is limited to about a dozen. 



FIG. 133. 



Median longitudinal section of sporogonium 

 of Andreaea rupestris, at the time of division 

 of the archesporium. f> = pseudopodium ; _/"= 

 foot; z; = vaginula; h = neck; c = columella; 

 w = wall of sporogonium j e = epidermis; s = 

 spore sac; *=archesporial cells dividing; 

 r=calyptra; s = neckof archegonium. (After 

 -Kuhn.) X8o. 



