100 Evans AND HOOKER: PERISTOME IN CERATODON PURPUREUS 
The first developmental study of the peristome in Ceratodon 
purpureus was made by Lantzius-Beninga (’50, p. 574), who com- 
pared it with the peristome of Trichostomum tortile, now known as 
Ditrichum tortile. In his figures of young transverse sections 
(50, pl. 66, f. 40, 41) he indicated the two layers of cells 
which give rise to the peristome but represented each layer as 
being composed of sixteen cells, those of the inner layer alternating 
with those of the outer. This was proved to be incorrect by 
Kienitz-Gerloff (’78, p. 44), who stated that, while the outer layer 
showed sixteen cells in section, the inner showed from twenty to 
twenty-four cells and that normally two cells of the outer layer 
bounded three cells of the inner. He showed further that the 
peristome developed in the amphithecial portion of the sporophyte 
and that the peristomial cells represented the fourth and fifth 
layers from the outside except in the region of the operculum, 
where they represented the third and fourth layers. The studies of 
Lantzius-Beninga and Kienitz-Gerloff have to do mainly with the 
region where the peristome is formed and give but few details 
about the actual development of the teeth, and the same thing is 
true of most of the work which has been done upon the peristomes 
of other Haplolepideae. 
In the present paper the account of the peristome will be 
divided into two parts. In the first the origin and development 
of the two peristomial layers will be described; in the second the 
deposition of the thickenings which constitute the bulk of the 
teeth will be considered. ae 
DEVELOPMENT OF THE PERISTOMIAL LAYERS 
As earlier writers have pointed out, the divisions which take 
place in the segments cut off from the apical cell of the young 
sporophyte proceed with great regularity, whether the segment is 
destined to form a part of the stalk or of one of the regions of the 
capsule. This regularity is characteristic not only of an indi- 
vidual species of the Bryales but of the group taken as a whole. 
~ 
It has been recently emphasized by Kuntzen (712) in the particular | 
case of Ceratodon purpureus and becomes strikingly evident 
through the study of the operculum and peristome. The.oper- 
cular segments, like those which go to form the spore-case, are in _ 
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