282 BRYOPHYTA 



fertile region of the capsule, the original formation is the same : it is 

 based upon an amphithecium, capable as we see of variously extended 

 development, and an endothecium which is less variable, though it may 

 expand also to form a more bulky tissue for water-storage : its most 

 distinctive function, however, is to give rise to spores, while below it 

 serves for conducting purposes. 



Above the fertile region the endothecium as a rule develops only cells 

 similar to those of the columella below : the amphithecium, however, 

 undergoes changes of induration of the walls, variously distributed, which 

 result in the formation of the operculum, the annulus by which it is 

 detached, and the peristome which is laid bare when the operculum falls 

 away. The details of the peristome may vary considerably in different 

 Mosses, but in all cases it appears to take its origin from the innermost 

 layer of the amphithecium. 1 The columella may in some cases co-operate 

 with it mechanically in the function of spore-distribution, but neither the 

 endothecium nor any of its products take any share in the development of 

 the peristome. 



It thus appears that the sporogonium of the stegocarpic Bryales is 

 composed of two tissue-tracts, distinguished early from one another in 

 segmentation, and divergent in their later development. The outer is 

 always sterile, while the other is fertile only in part. The question arises 

 as to the initial condition, and the origin of these regions. It may be in 

 some degree elucidated by comparison of some of those smaller forms in 

 which the sporophyte is of simpler construction : they have in common 

 the feature that the mechanism of dehiscence is absent, or imperfect, and 

 on this account they have been grouped together as the Cleistocarpae. 

 It is clear at the outset that this condition may either have been 

 primitive or the result of reduction : these alternatives must be kept in 

 mind in any discussion of such forms, even though no definite conclusion 

 be arrived at. 



(b) Cleistocarpae. 



Of the various genera grouped as the Cleistocarpae, Phascum has been 

 examined developmentally by Kienitz-Gerloff : the primary segmentation is 

 according to the type of the Bryales, with well-marked endothecium and 

 amphithecium (Fig. 139). From the periphery of the former the arche- 

 sporium is derived in the usual way, while the formation of an air-space 

 and spore-sac, and the enlargement of the cells of the columella, are all 

 according to the usual type : stomata may also be present, but there is 

 neither operculum nor peristome. Developmentally there is a near similarity 

 to Andreaea, though on the ground of its peculiar dehiscence and domed 

 archesporium this genus is usually kept apart. It has, however, been 

 pointed out above that a very slight modification of the ordinary type of 

 Andreaea would produce the condition of the archesporium seen in 



1 Goebel, Organography, p. 383. 



