324 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



Dhought-hesistant plant breeding. — The best of the selected 

 strains of alfalfa developed at Bellefourche, S. Dak., in the course of 

 this work continue to show groat uniformity in their desirable forage 

 type and in seed production, both at Bellefourche (S. Dak.) and at 

 Akron (Colo.). This uniformity has been particularly noticeable in 

 connection with one of the strains, seed of which was grown at both 

 places. The very dry weather of June. 1010, has given the first severe 

 test of drought resistance which has been had during the last three 

 years. Most of the selected strains have so far stood the test satis- 

 factorily. If the conditions of drought continue, an adequate test 

 of the resistance of the improved strains of sorgo and millet that 

 have been bred can be made. A study of several hundred individual 

 plants of Bromus inermis^ grown in hills in the breeding nursery at 

 Bellefourche, showed that a number of distinct types could be selected, 

 one of which is especially satisfactory as a forage plant. This grass is 

 showing remarkable resistance to the prevailing conditions of drought, 

 and considerable difference in this respect is manifested by the differ- 

 ent types. Further study of the individual plants of western wheat 

 grass in the nursery shows abundant opportunity for the selection of 

 desirable forage types of this highly drought-resistant native grass. 

 Seed of such of the new strains of the crops mentioned as prove to be 

 definitely superior in drought resistance will be increased as rapidly 

 as possible for distribution in the regions to which they are adapted. 



Physiology of drought resistance. — Under the immediate direc- 

 tion of Dr. H. L. Shantz, Physiologist, and in cooperation with the 

 Physical Laboratory, numerous varieties of most of the important 

 crop plants grown in the Great Plains area are being studied in order to 

 ascertain what structural and physiological peculiarities make some 

 species and varieties more drought resistant than others. Parallel 

 work is being carried on with the native plants of the region, in which 

 the drought-resistant characteristics are usually more easily dis- 

 tinguished. It is found that the best adapted of the cultivated plants, 

 as well as most of the native plants, fall into two classes — those which 

 endure drought and those which merely escape it. 



Experiments with a large number of species and varieties seem to 

 indicate that there is little difference in the ability of the roots to take 

 up water from a given t^q^e of soil when nearh'- dry, the minimum 

 percentage of soil moisture permitting the roots to absorb being nearly 

 the same for all the crop plants investigated. Among plants which 

 are really drought resistant the two principal factors appear to be 

 ability to withstand wilting by reducing transpiration in times of 

 drought, and ability to recover and continue growth after wilting, 

 when water again becomes available. 



It is found that many plants which are general^ considered 

 drought resistant merely avoid drought by making a comparatively 

 small growth of leaves and stems, thus conserving soil moisture. 

 Other plants, by ripening their seeds rapidly, escape the extremely 

 dry weather of the latter part of the summer. Another adaptation by 

 which some apparently drought-resistant plants in reality escape 

 drought conditions is by the development of deeper or more extensive 

 root systems, which enable them to draw water from a larger volume 

 of soil. 



