Evans AND Hooker: PERISTOME IN CERATODON PURPUREUS 101 
the form of half-cylinders, which become divided into quadrants 
by anticlinal walls. Cross sections through young sporophytes 
just below the apical cell show this condition clearly (Fic. 1, 2). 
It is in the quadrants that the division takes place according to 
the “fundamental square’”’ method. If transverse walls are left 
out of consideration each quadrant divides by an anticlinal wall, 
extending as a curved surface from one of the side walls to the 
outside wall, into two cells (FIG. 3), a smaller triangular cell (as 
seen in section) and a larger four-sided cell. The latter soon 
divides by a periclinal wall into an outer and an inner cell (FIG. 4). 
According to Kuntzen (12, p. 19) the segments in the stalk 
sometimes divide in the way just described and sometimes by 
means of a periclinal wall followed by an anticlinal wall in the 
outer of the two cells thus formed. In the spore-case he finds the 
second method only but admits that perhaps the first method is 
sometimes followed. In either case, after the rearrangement of 
the walls, the cross section shows four inner cells, the ‘‘funda- 
mental square,’’ surrounded by eight outer cells. The inner 
cells constitute the endothecium and the outer cells the am- 
phithecium. A similar arrangement of the cells can be demon- 
strated in the young sporophytes of most of the higher bryophytes. 
In the next stage of development the amphithecial cells divide 
by periclinal walls, thus giving rise to two layers of eight cells 
each (FIG. 5). The inner cells, shaded in the figure, will develop 
into the inner peristomial layer. The corresponding layer in the 
spore-case gives rise to the outer spore-sac. It is upon the 
periclinal walls separating the two amphithecial layers that the 
teeth of the future peristome will be deposited. Marked differ- 
ences occur in the endothecium and amphithecium with respect 
to the relative rate of cell division. In the region of the annulus 
the inner peristomial layer completes its divisions before the 
endothecium has passed beyond the four-celled stage. In the 
region of the peristomial teeth the division may be simultaneous 
or the endothecium may divide first. In the portion of the 
operculum above the peristome the inner amphithecial layer never 
passes beyond the eight-celled stage. 
In the region of the teeth, after the stage shown in FIG. 5, 
the cells of the outer amphithecial layer divide by anticlinal walls, 
