Physiological Anatomy of Xerophytic Selaginellas. 107 
practically parallel with the stem. Only during the very hot 
summer when the temperature rises as high as 41*5° in the sun 
and 30° in the shade, can one recognise the advantage of this 
feature, when during the hottest hours of the day the sun’s rays 
fall parallel with the leaves, whereas if the leaves stood horizontally 
they would be exposed to the perpendicular incidence of the rays. 
Further, one observes that the greyish-blue cuticle of the epidermis 
reflects the light to a certain extent, although this feature is not so 
marked as in the species of Group III. The growing point is 
protected by a close bundle of long awns on the young leaves 
surrounding the apex of the stem. The cells of these awns are 
elongated, thick-walled and entirely destitute of chlorophyll, 
reflecting almost all the light falling upon them (Fig. I, 5 ; II, 3, 4, 5 ; 
III, 9; V, 5). The same feature can be observed in all the species 
belonging to this group. Awns on leaves of older and lower parts 
of the stem do not seem to be of great importance, as a protection 
from the sun, but they retain the power of reflecting light. In 
S. densa the leaves are broader than those of other species (Fig. II, 
5) , and are folded length-wise round the stem, the halves being 
almost at right angles to each other, and lying very close against 
the stem. This structure and position of the leaves minimises trans¬ 
piration. 5. densa was collected in a dry pine wood in Ipappatah, 
Florida. The same conditions exist in psammophilous species, 
i.e.y those found in sandy soils, which are for a large part of the 
year physically dry, on account of the rapid drainage of water 
after rains. Species from such localities are S. avenicola , 5. 
funiformis , 5. acanthonota and others. 
Sections of various parts of plants of this group exhibit a 
very sclerotic anatomical structure. A section of the stem (Fig. Ill, 
6) , of S. rupestris, collected on an exposed granite rock near Sabino 
Canon, shows a very thick cuticle; the cell cavities of the 
epidermis being very small, on account of the secondary layers of 
thfe walls. The hypodermis is usually composed of three rows of 
cells of the same size as those of the epidermis. The cells of the 
cortex are larger, and the walls become gradually thinner as one 
proceeds further into the plant, although they are never as thin as 
those of hygrophytic forms. In thin sections one readily finds in 
the thick walls layers and remains of plasmodesmi. The 
protoplasm of the epidermis, hypodermis and outer cortical cells 
very soon dies, after the secondary wall layers are formed. In 
very young tissues these cells sometimes contain a few chloroplasts, 
