MUSCI. 
3«3 
on all sides by the spore-sac, is distinguished as the Columella. The spore-sac is ruptured 
by the casting off of the operculum, but the columella remains dried up, and in Poly- 
trichum there remains also a layer of cells, the Epiphragm, attached to the points of the 
teeth of the peristome, and covering the opening of the theca. 
We must now examine somewhat more closely the origin of the peristome. In 
those genera which, like Gymnostomum, do not form a peristome, the parenchyma 
which lies immediately beneath the operculum is homogeneous and thin-walled; when 
the theca is ripe, it contracts and dries up within the operculum, which is formed 
essentially only of the epidermis ;. or it remains attached to the columella and forms 
a thickening at its summit, which projects over the opening of the theca; or again 
it forms a kind of diaphragm, which closes the mouth of the theca after the casting 
off of the operculum {Hymenostomum). The transition to the genera provided with 
a true peristome is furnished by Tetraphis. In this genus the firm epidermis of the 
upper conical part of the theca falls off as the operculum, while the whole of the tissue 
immediately beneath the operculum, the two outer layers of which are thick-walled, 
splits across into four valves. These are also termed by systematists a peristome, 
although their origin and structure are widely different from that of the true peristome 
in other genera. For, except in the Polytrichacese, neither the teeth nor the cilia con- 
sist of cellular tissue, but only of thickened and hardened parts of the walls of a layer of 
cells, which is separated by some layers of thin-walled cells from the epidermis which 
forms the operculum ; the latter layers, as well as the delicate parts of the former, 
become ruptured and disappear, while the thickened parts of the wall remain after 
the casting off of the operculum. This will be rendered clear by an example. Fig. 
271 represents a part of the longitudinal section which bisects the theca of Funaria 
hygrometrica symmetrically, corresponding to the part in Fig. 266, C, designated a] 
e e \s the reddish brown epidermis strongly thickened on the outside ; at the part where 
it bulges its cells are of a peculiar shape, forming the ring or annulus ; se is the tissue 
lying between the epidermis of the theca and the air-cavity h ; the large-celled tissue p 
is the prolongation of the columella into the cavity of the operculum ; at S are seen 
the uppermost spore-mother-cells ; directly above the air-cavity h rises the layer of cells 
which forms the peristome ; its walls [a], which face outwards, are strongly thickened, 
and of a bright red colour ; the thickening is continued also partially along the septa ; 
the longitudinal walls which lie on the axial side of the same layer of cells (/) are also 
coloured, but less strongly thickened. In Fig. 272 is shown further a part of the 
transverse section through the basal part of the operculum ; r r are the epidermal cells 
placed immediately above the annulus, forming the lower edge of the operculum ; a 
and i the thickened parts of the layer of cells concentric with the operculum, which 
form the peristome. A section near the apex of the operculum would show, instead 
of the broad thickening-masses /, i', i" , only the middle part of the inner wall, but 
more strongly thickened. If now it is supposed that when the theca is ripe the annulus 
and the operculum fall off, the cells/* and those which lie between a and e (Fig. 271) 
disappear, and that the thin pieces of wall between a', a", and between /, i', i", in Fig. 
272, are also destroyed, then the thick red pieces of wall alone remain, forming sixteen 
pairs of tooth-like lobes pointed above, crowning the edge of the theca in two concentric 
circles. The outer row are termed Teeth, the inner row Cilia. The thickened cells at 
t, Fig. 271, unite the base of the teeth with the edge of the theca. According as 
the layer of cells which forms the peristome consists, in transverse section, of a larger 
or smaller number, and according as one or two thickened walls are formed within each 
one of these cells, the number of teeth and cilia varies; it is always however a multiple 
of four, generally 16 or 32. In niany cases the thickening at i is wanting; the peri- 
stome is then simple, and formed only of the teeth of the outer row. The thickenings 
at a are very commonly much stronger than is the case in Funaria, and the teeth there- 
fore stouter. The thickened parts of the wall may also partially or entirely coalesce 
laterally with one another ; and then the parts of the peristome either above or below 
