PLANT SOCIETIES 317 
(Fig. 245). The epidermis, even on the younger portions 
of the stem, is highly cutinized (Fig. 121), and this structure 
makes any evaporation very slow. 
‘387. Leaves of Xerophytes.—In regions where the 
greatest dangers to vegetation arise from long droughts 
and the excessive heat of the sun, the leaves of plants 
usually offer much less surface to the sun and air than is 
the case in temper- 
ate climates. Some- 
times the blade of 
the leaf is absent 
and the expanded 
petiole answers the Ce 
purpose of a blade, Ce Aa ey eat 
MASes 
or, again, foliag € ia. 224.— Cross-Section of Rolled-Up Leaf of Crow- 
leaves ar e alto- Berry (Empetrum nigrum). (Magnified.) 
gether lacking, as in the cactuses (Fig. 222), and the green 
outer layers of the stem do the work of the leaves. 
» 388. Rolled-Up Leaves. — Leaves which receive but a 
scanty supply of water are often protected from losing it 
too rapidly by being rolled up, so that the evaporating, 
t.e., stomata-containing, surface is on the inside of the roll. 
Sometimes, as in the crow-berry. (Fig. 224), the curled con- 
dition is permanent. In other plants, as in such grasses 
as Stipa (Fig. 225), and in Indian corn, the leaf rolls up 
when the weather is very dry and unrolls again when it 
receives a better supply of water. 
389. Mesophytes.— A mesophyte is a plant which 
thrives best with a moderate supply of water. The great 
majority of the wild and the cultivated plants of the 
United States are mesophytes, and what has been learned 
