12 THE FUTURE OF ARID LANDS 
matter—a variation of more than seven times (17). This range for 
different plants is about the same as the difference measured be- 
tween the same plant grown in a semi-arid and an extremely arid 
climate. The grain crops grown most successfully in the arid 
zone are of this group. They have a short season, rapid growth, 
large grain yield in proportion to the straw, and a low water 
requirement. 
3. The term drought resistance can be reserved for the suc- 
culents, which impound water within their plant body and are 
able to continue growth when soil moisture is entirely unavailable. 
They are admirably adjusted to absorb soil moisture rapidly as 
soon as it is available in light showers and to store it in their 
bodies. They are not characteristic of the extreme deserts. 
4. Drought enduring plants estivate when drought occurs and 
they can resume growth as soon as water is again available. In 
other words, they become drought dormant. They also possess 
most of the characteristics of the drought evading plants by which 
they are enabled to prolong the growth period. The most im- 
portant and prominent desert plants belong to this group. When 
soil moisture is no longer available, these drought enduring plants 
become drought dormant or estivate in the vegetative stage, 
while the drought escaping plants pass through the drought only 
in the seed stage. Many bacteria, fungi, algae, lichens, mosses, 
and ferns are able to endure drought. 
Great adjustments are made by plants to enable them to grow 
in the arid lands. Great as are the physiological Fences 
they are small compared with the variation in size shown by the 
drought escaping plants which will still ripen seeds when less than 
one thousandth of normal size. 
Animal Adjustments to Arid Conditions 
Animals show as great adjustments as plants do to enable 
them to survive in the arid lands (15). They too developed in 
water and they had to develop lungs or tracheal systems in order 
to secure oxygen from the air. They must also have a constant 
supply of water for their active growth. One of the important 
reasons why animals can live in the arid zone is found in their 
