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 cf the amphithecium.!. 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 6f such forms, even though no definite conclusion 
be arrived at. 
(6) Cletstocarpae. 
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 
1Goebel, Organography, p. 383. 
