STEVENS: DISCOID GEMMAE IN LEAFY HEPATICS 371 
transverse walls, but the segments which have arisen directly from 
the division of an apical cell may generally be traced by their 
position in the mature gemma. In gemmae having two apical 
cells one may continue to function as an apical cell after the other 
has lost its power of regular segmentation and has begun to divide 
by periclinal and anticlinal walls (FIG. 3, D). This gives rise to 
still greater irregularity in the mature gemmae. The usual posi- 
tion of the mature gemmae, at right angles to the plane of the 
leaf, seems to be due to the crowding of the gemmae as they in- 
crease in size. For the gemmae are at first in the plane of the 
leaf and remain so in places where they are not close together. 
x 
Cc D 
FIGURE 3. Radula complanata. A. Gemma with one apical cell. B-D. 
ae in which the apical cells have ceased to cut off regular segments and have 
divided irregularly. XX 300 
The active division of an apical cell tends to inhibit the division 
of other cells in the gemma. This appears from the fact that when 
one apical cell is present the portion lacking an apical cell exhibits 
very little cell division. It is shown still more strikingly, however, 
by the fact that about the time the apical cell loses its power of 
active segmentation the older cells of the gemma begin to divide 
by walls parallel to the surface. 
A mature gemma, that is, one ready to fall from the leaf is thus 
typically two cells thick for a considerable part of its extent (FIG. 
