40 
F. Cavers. 
dispersal, and this attains its highest development in the typical 
double peristome of the Diplofepideae, the dominant Moss group 
and the culminating point of Bryophyte evolution. Comparison of 
a large number of Bryales shows that in general there is a definite 
relation between the size of the spores and the diameter of the 
apertures between the endostome processes and cilia—the smaller 
the spores the more numerous and completely developed are the 
cilia of the endostome. 
In the higher Mosses, the tubular or barrel-like form of the 
archesporium, which is open above and below, allows ready passage 
of food materials to the apical region of the capsule, and in this 
region considerable growth occurs, leading to the development of 
annulus, operculum, and peristome. We may perhaps assume that 
the Tetraphis type of peristome is the most primitive, and that it is 
derived from that seen in those species of Andrecea in which the 
slits in the capsule-wall extend to the apex of the capsule and thus 
lead to the formation of a series of teeth. In Tetraphidace^e the 
entire tissue of the capsule apex takes part in the formation of the 
simple peristome, the epidermal layer itself separating from the 
underlying tissue to form an operculum only after it has become 
slit into four lobes at the base. In the Polytrichales, we find that 
the development ol the peristome is confined to a zone of amphi- 
thecial cells surrounding the columella; in the more primitive 
Dawsonia type this zone is several cells deep, while in the more 
advanced Polytrichum type it is limited to a single layer of cells. 
In the Buxhaumia type, again, the peristome-forming zone is broad, 
and the number of teeth forming the exostome is indefinite, these 
teeth being arranged in several concentric series. Finally, in the 
Eu-Bryales the peristome arises in a sharply defined zone of 
amphithecial cells and the teeth are always in two series. Along 
with this limitation of the peristome-forming zone to fewer and 
fewer layers of cells, we find a transition from the primitive 
Tetraphis type in which the peristome teeth consist of cell-masses 
to the Bryum type in which it consists solely of the remains of 
thickened and persistent cell-membranes. 
The phylogenetic views thus briefly and tentatively put forward 
may be expressed diagrammatically in the accompanying table. 
