27 
Bryales. 
layer of the endothecium (in Archidium from the entire endothecial 
tissue) and is a tubular zone, hence the columella extends into the 
apex of the capsule ; ripe capsule usually opens by a lid, which in 
the higher types is detached by the separation of a ring of special 
cells and not merely by splitting of cell-walls, and a special 
mechanism (peristome), which ensures gradual spore-dispersal, is 
usually present and is typically developed from the inner amphi- 
thecial tissue lying above the archesporial region of the capsule. 
These three groups are closely connected with each other, and 
sharply separated in many respects from the lower Bryophytes. 
The three “ Moss ” groups agree closely in the vegetative characters 
of the gametophyte, in the growth of the young archegonium by 
means of an apical cell, and in the absence of sterile cells or elaters 
from the -archesporium. 
The Bryales, by far the largest group of Bryophyta, present 
extraordinary uniformity in the early development of the sporo- 
gonium and the mode of origin of the sporogenous tissue. 
Comparatively little work has been done on the embryogeny of the 
group, but it would appear that in general the archesporium arises 
from the outermost zone of the endothecium, and the outer spore- 
sac from the innermost zone of the amphithecium, the characteristic 
air-space of the Bryinean capsule being formed between the outer 
amphithecial tissue (which produces the capsule-wall) and the outer 
spore-sac, while the inner spore-sac consists of a layer of cells cut 
off from the inner side of the primary archesporial layer. The only 
outstanding exception to this general and probably universal mode 
of origin of the sporogenous tissue is that seen in the genus 
Archidium ; here the air-space is formed as in typical Bryales, but 
there is no definite archesporium, unless we regard the entire 
endothecium as representing the archesporium ; at any rate, there 
is no differentiation into columella and archesporium proper, and a 
few of the endothecial cells become spore-mother-cells, the sterile 
cells among which the fertile cells are scattered being absorbed 
during the maturation of the spores. In Ephemerum the embryogeny 
follows the normal Bryinean type, but the central sterile tissue of 
the endothecium produces a relatively feeble columella, which in 
some species disappears before the capsule reaches maturity. In 
the allied genus Nanomitrium, the slender columella is used up in 
the ripening of the spores, but the capsule has a zone of cells which 
form a rudimentary annulus and delimit the upper portion of the 
capsule as an operculum. 
