10 THE FUTURE OF ARID LANDS 
suffer or die because of lack of oxygen. If they are watered from 
below, saturated soil or a water table is established at the bottom 
of the container. Therefore all experimental containers a few feet 
deep in which the experimenter expects the excess of rainfall or 
irrigation water to drain out will find only hydrophytic or swamp 
conditions which do not even approximate field conditions. 
Let us consider some of the many adjustments xerophytes 
have made to enable them to survive and to grow in the arid 
lands. They must be able to grow under the following conditions. 
The climate varies from semi-arid to extremely arid. The relative 
humidity is often as low as § to 35%. The evaporation rate is high. 
The soils are pedocals moistened only at the surface and with a 
permanently dry sub-soil. 
The roots of these plants are extensive but limited to the soil 
moistened by the precipitation. The root surface is large as com- 
pared with the aerial plant and root hairs are abundant. 
The stems are small, often with deciduous leaves or none. 
Cladophylls or bracts or stems often function as leaves. The 
branches are often crowded together or are underground for 
protection. 
The /eaves are generally thick, small, firm, and leathery (sclero- 
phyllous); they do not wilt but endure drought and recover when 
water 1s again available. Leaves are often deciduous. 
The internal structures also show many adjustments. 
The roots of xerophytes often have cork or sand sheaths near 
the soil surface. 
The stems have cork or thick cuticular coverings. The stems 
often function as leaves, especially when leaves are absent, and 
have several layers of palisade tissue. The stomata are sunken 
or protected. 
The /eaves, if present, have the leaf lamina reduced or absent 
and the cells and stomata are small. They are sclerophyllous with 
much supporting tissue, do not wilt, but often roll or fold to pro- 
tect the stomatal areas. The outer epidermal walls have thick 
cuticles with waxy or resinous coating. The stomata are sunken 
and protected. The epidermis often has woolly, scaley, or stellate 
hairs filled with air to reflect light. The leaves have several layers 
