338 WILLISTON: DISCOID GEMMAE IN RADULA 
differentiated peripheral growing points, and no gemmae were 
found in which the leafy shoot had begun to appear. Several 
young plants showed thalloid fragments clinging to them, but it 
was impossible to determine in any of these cases whether the 
fragment was a part of the gemma itself, as in R. flaccida, or 
part of a thallus such as is formed in R. Hedingert. 
RADULA LINDBERGIANA AND RADULA SUBTROPICA 
R. Lindbergiana, a species native to Europe, Asia, and Algeria, 
first described by Gottsche in 1864, and R. subtropica, native to 
Brazil, and described by Stephani in 1910, were also studied. In 
these the gemmae were essentially the same as those of R. com- 
planata as figured by Stevens (’10, p. 370). 
COMPARISON OF SPECIES 
Gemmae in the genus Radula may be arranged in two groups 
according to their complexity. The first group contains the 
simplest types and has representatives in three species, R. com- 
blanata, R. Lindbergiana, and R. subtropica. The gemmae in this 
group occur on the margins of leaves and are very irregular in 
size and shape when mature. This irregularity is due to the 
variation in the number and activity of the apical cells and their 
method of development. These gemmae may be more than one 
cell thick. Germination in this group has not yet been described. 
The second group contains the more complex types and has 
representatives in four species, R. protensa, R. Hedingeri, R. 
tiibodensis, and R. flaccida. These gemmae occur not only on 
the margins but also on the surface of leaves and follow a more 
regular course of development, the mature gemmae in a given 
species being uniform in shape and size. In R. flaccida and & 
protensa apical cells are present, but R. Hedingeri and R. tjibodens’, 
while developing after a fixed plan, do not, according to Goebel 
('87, p. 62), possess any apical cells. All the gemmae belonging 
to this group are only one cell thick. Germination in this group 
may be divided into two categories. In the gemmae of R. Hedew: 
gert, and R. tyibodensis, as described by Goebel, the procedure 1s a 
follows: The circular gemma develops at some point on the margin 
a thalluslike body as large as the gemma itself. One of the 
marginal cells of this thalloid outgrowth in turn gives rise to the 
leafy shoot, but by the continued growth of the tissue adjoum"s 
