WILLISTON: DIsSCOID GEMMAE IN RADULA 331 
though groups of seven or eight may occur. In instances where 
more than one are produced on a single leaf, usually only one 
matures, and the others either become abortive or delay their 
development until the first one is shed. 
In the development of the gemmae the sequence of cell division 
can be traced accurately only in the earlier stages, as with the 
increasing complexity of the gemmae the order of cell formation 
becomes somewhat variable. _ The formation of a gemma begins 
by the enlargement of a single marginal cell and its protrusion 
beyond its neighbors. This cell begins at once to secrete a trans- 
parent gelatinous substance. A similar secretion was noted in 
the early stages of R. complanata by Stevens (’10, p. 369), who 
suggests that the presence of this gelatinous substance may be 
taken as a rough indication of the region of most rapid growth. 
As this cell increases in size and pushes farther beyond the margin 
of the leaf, it takes on a knoblike shape (FIG. 5), and finally the 
end of the knob is cut off by a wall parallel to the margin of the 
leaf (FIG. 6). The inner of the two cells thus formed is the stalk 
cell, the outer the mother cell of the gemma. The stalk cell 
undergoes no division but increases slightly in size. The outer 
cell, however, soon divides by a median wall at right angles to 
the first. Each of the resulting cells then divides by a transverse 
wall. The original mother cell is now separated into quadrants (FIG. 
7). The two outer quadrants are triangular in shape and generally 
begin at once to function as two-sided apical cells, cutting off seg- 
ments first on the side toward the median line then on the side 
toward the lower quadrant, and so on until three or four segments 
ids been formed (FIG. 8, 9, 10, 11, 12). The earlier segments 
divide first by periclinal walls and later by anticlinal walls, so 
that by the time the last segment is cut off by the apical cell the 
first may have divided into as many as six or eight cells. The 
4Pical cells both cease to function after a short time; and instead 
of Cutting off segments in the normal way each divides by a 
Periclinal wall followed by an anticlinal wall in the peripheral 
cet, thus giving rise to three cells. 
The establishment of the apical cells is not always simultaneous 
on the two sides of a single gemma. The original triangular 
Wadrant on one side or both may cut off an extra cell parallel 
