Physiological Anatomy of Xerophytic Selaginellas . 125 
are all large and thin-walled in the hygrophytic forms, and 
thick-walled and small in the xerophytic ones, as can be seen in 
comparing *S. pilifera (Fig. VII) and 5. Galeottei , a hygrophyte 
(Fig. XII). The hypodermal cells of the latter are, however, thicker 
walled, than the cortex, but do not contain any suberin or only 
to a slight extent. The cuticle contains suberin. The difference 
in width of the xylem vessels and cortex cells is given below. 
Width in ft of vessels of xylem and of cortical cells in Selaginellas. 
Name 
Environment 
Width of 
vessels 
of xylem 
Width of 
largest 
cortex cells 
Hygrophytic Plants. 
S. Galeottei 
moist greenhouse 
40-70 
80 
Xerophytic Plants, 
5. pilifera 
from desert 
10-20 
50 
S. pilifera 
from moist green¬ 
house 
20-30 
62 
S. lepidophylla 
from desert 
10-20 
52 
S. cuspidata 
from desert i 
10-20 
50 
S. P ring lei 
from desert 
15-30 
55 
S. sanguinolenta 
from exposed rocks 
8-10 
30 
S. rupestris 
from exposed rocks 
20-35 
40 
S. rnpestris 
from greenhouse 
25-42 
45 
S. arenicola 
from sand barrens 
20-30 
40 
S. densa 
from exposed rocks 
18-30 
35 
S. rupincola 
from exposed rocks 
20-30 
40 
The occurrence and number of stomata, which regulate to a 
considerable extent the transpiration of plants and the movement 
of water, are worth consideration. These stomata are all protected by 
a thick cuticle, which in species of Group I has a special air chamber, 
followed by a narrow passage through which the entrance between 
the two guard cells is reached. The situation of the stomata on 
the leaves is peculiar to each of the three groups. The most occur 
as a rule toward the midrib of the aligular surface of the dorsal and 
ventral leaves, since most air-spaces of the spongy mesophyll are 
to be found in this neighbourhood. The few stomata which occur 
on the ligular surface of both leaves have an unimportant 
role. The author in the following table summarises the presence 
of stomata in the plants with which he worked. 
The principal question which arises after having studied 
xerophytic Selaginellas is the problem of the immediate protection 
and behaviour of the protoplasm of plants in desert and other 
semi-arid situations during extreme drought. Examining cells of 
above plants one notices a considerable number of large oval and 
round drops which ooze out of the open cells, when sections are 
