130 
/. C. Th. Uphof. 
a thick-walled epidermis; the wings contain a well developed 
sclerotic tissue, all heavily suberized. The elements of the proto- 
and metaxylem are considerably narrower than those of hygro- 
phytic species. Plants which were grown in a moist shady green¬ 
house develop horizontal leaves; the various tissues are neither as 
thick-walled nor as heavily suberized; the xylem has wider vessels. 
Some species are petrophilous, other are decidedly psammophilous. 
The second group is composed of plants with a spreading 
habit; some show affinities with the last group, since they are able 
to roll up during drought. The anatomical construction is not 
as sclerotic as in the previous group, although the walls of the 
epidermis, hypodermis and outer cortex are very thick-walled. 
Some species produce a red pigment which is deposited in the cell 
walls of very exposed stems. In some forms the vessels of the 
xylem are very narrow, which is correlated with the low water 
supply and the very small leaves of the plant. The cuticle and 
other walls of the epidermis of the leaves are extremely thick. 
The mesophyll is well developed and possesses large air-spaces in 
the spongy parenchyma. 
Plants of the last group are exceedingly well fitted for a semi- 
arid environment. The stems are densely tufted and curl up into 
a more or less close cluster; the leaves are placed horizontally on 
both sides of the stem. 
During intense heat the exposed parts of the stem and leaves 
reflect the light considerably ; these parts are white or brown to 
red-brown, according to their exposure to the heat. The brown 
color is always in the cell walls. On very exposed places the 
underside of some species is protected by a colloidal layer against 
extreme transpiration, and this retains the water for a long time 
when moistened. 
The anatomical construction of all species shows a rather thin- 
walled hypodermis and cortex on the upper (inner) surface of the 
stem, and a thick-walled hypodermis and cortex on the lower 
(outer) surface. During drought the thin-walled tissue loses more 
water than the thick-walled tissue, which causes the plant to curl 
up ; it absorbs more water when moistened which causes the plant 
to spread out. The statements of Leclerc du Sablon and Wojinorvic 
are misconceptions. These investigators state that the thick- 
walled tissues are on the upper side of the stem and absorb more 
water than the thin-walled tissues when moistened, and lose more 
water when drying. The hygrochastic movements are mechanical 
and can be also observed on dead plants. The plants increase in 
