104 Evans AND HooKER: PERISTOME IN CERATODON PURPUREUS 
while the cells of the outer layer divide by anticlinal walls in a 
more or less indefinite manner. When the capsule approaches 
maturity the endothecial tissue within the peristome consists of a 
relatively small number of large and thin-walled cells, this tissue 
of course being the continuation of the columella and the sporog- 
enous tissue in the spore-case. 
While the segments have been undergoing the anticlinal and 
periclinal divisions just described, divisions by transverse walls 
have also occurred in the various layers. Apparently they occur 
in a rather indefinite way and vary in different parts of the capsule 
(see Kuntzen, ’12, p. 22, etc.). In the peristomial layers trans- 
verse walls are especially numerous toward the base and tend, in 
this region at least, to be more numerous in the outer layer than 
in the inner (FIG. 13). Toward the apex they are more numerous 
in the inner layer (FIG. 12). 
DEPOSITION OF THE PERISTOMIAL THICKENINGS 
The forty rows of cells taking part in the formation of the 
peristome may be naturally divided into eight groups of five rows 
each. In each group three rows of the inner peristomial layer 
correspond with two rows of the outer layer. A cross section of 
such a group before the deposition of thickening is shown in 
FIG. 11. A group gives rise to two teeth of the peristome and the 
Same processes are repeated in each group. The peristomial 
cells, before the thickenings are laid down, contain dense cytoplasm 
and small nuclei with several nucleoli but are usually without 
vacuoles (FIG. 11-13). In the upper part of the peristome, 
where the teeth are divided into branches, eight regions of thicken- 
ing can be distinguished in each group, the four of the outer layer 
corresponding with the four of the inner layer. Two regions 
belong to each cell of the outer layer and to the median cell of the 
inner layer, while one region belongs to each lateral cell of the 
inner layer. The thickening first makes its appearance in the 
inner layer (FIG. 14) but soon becomes evident in the outer layer 
as well (FIG. 15, 16). If two corresponding regions are regarded 
as forming a single strand, each group will then form four strands, 
_ representing the four branches of the two peristomial teeth. 
The deposition of thickening begins in the basal portion of 
