SCIENCE IN MAN'S STRUGGLE ON ARID LANDS 39 
confidently expect continued improvement in arid land manage- 
ment through our ability to remove the useless plants that waste 
water so that we may conserve it for the good ones. 
Despite this progress, I feel that plant breeding will make as 
great or even greater long-time improvement. Most of our out- 
standing results from breeding new varieties, in contrast to selec- 
tion alone, have been of plants to be grown under favorable condi- 
tions of soil and water. Crops grown under irrigation in 
arid regions, such as cotton, alfalfa, corn, and potatoes, have been 
greatly improved through breeding. In unirrigated arid regions, 
however, we shall be breeding plants for growth under severe limi- 
tations of moisture. 
We know that most of our plants are highly inefficient in their 
use of water and light. As plants proceed with their basic function 
—the manufacture of plant food by photosynthesis in sunlight— 
they are required to take in carbon dioxide. In most of them, as 
they take this in, water can escape. But the extent of this water 
loss varies widely. Basic researches on these processes can give our 
plant breeders new plant materials. Some plants can even take in 
large amounts of water from humid air, such as that over the 
plants during a cool night. Some plants can fix nitrogen, or at 
least sustain other organisms that can. It is know known that this 
ability is not limited to legumes like clover and alfalfa. 
If characteristics of plants such as drought resistance, drought 
tolerance, and low water requirement can be related to specific 
genetic patterns, I see no reason why the plant breeder cannot 
combine them with other desirable characteristics as he has 
already done with disease resistance. 
Thus besides selection and testing of plants collected from arid 
lands, I feel that the basis for breeding plants for arid conditions 
has improved. Such breeding on a significant scale, except for 
lucky accidents, will be preceded by more fundamental research 
of the genetic basis for the primary growth factors in promising 
species and varieties. The newly emerging discipline, sometimes 
called “physiological genetics,” holds great promise for giving us 
far greater potentialities than can be had from plant selection 
alone. 
The long-time potentialities of such plant breeding, based on 
