BUREAU OF PLANT INDUSTRY. 143 



Water requirement of plants,— The study of the water require- 

 ment of plants has been continued during the past year with a view 

 to determining the crops and varieties most efficient in the use of 

 water. The differences exhibited in this respect by the principal 

 crop plants are remarkable. Millet, sorghum, and corn are the most 

 efficient in the use of water. Wheat and the other small grains form 

 an intermediate group, while alfalfa and other legumes have the 

 highest water requirement. Alfalfa uses about three times as much 

 water as millet in the production of a pound of dry matter when the 

 two crops are grown side by side. Varieties of the same crop show 

 in some instances marked differences in water requirement, so that 

 the careful study of different varieties from this standpoint is a 

 matter of decided economic importance in connection with the agri- 

 cultural development of dry-land regions. 



Mottle-leaf. — Investigations regarding the cause of mottle-leaf of 

 citrus trees in southern California have shown that about one-half 

 of the mottle-leaf can be accounted for by the low humus content 

 of the soil. The trouble appears also to be due in part to poor dis- 

 tribution of the irrigation water and a concentration of mineral 

 plant food in the topmost layer of soil, where it is useless to the tree. 



Indicator value of native vegetation. — Investigations in south- 

 ern Arizona and southeastern California demonstrated that in this 

 region certain types of vegetation indicate that the soil contains 

 too much alkali for successful crop production ; others indicate that 

 the land is not too salty to be easily reclaimed under irrigation ; and 

 still others indicate that alkali is absent or present only in negligible 

 quantity. The texture and moisture-holding capacity of the soil and 

 its suitability for the production of different classes of crops are also 

 indicated with a high degree of accuracy by the character of the 

 native growth. In view of the great areas of land in the Southwest 

 which are still in their original state but of which a portion is capable 

 of crop production, the establishment of these definite correlations is 

 believed to be of considerable practical importance. 



PLANT-BREEDING INVESTIGATIONS. 



Alfalfa. — The introduction and breeding of hardy and drought- 

 resistant strains of alfalfa are being prosecuted as vigorously as 

 possible. An especially promising new strain has been secured from 

 India, which seems to be well adapted for use in the Southwest. In- 

 vestigations to determine the cause of " yellowing " of alfalfa and to 

 develop a practical treatment for this trouble, which is a serious 

 handicap to the culture of the crop in many parts of the East, indi- 

 cate that the stage of maturity at which the crop is cut has much to 

 do with the degree to which the succeeding crop is affected. 



Clover. — The pollination studies of red clover carried on in co- 

 operation with the experiment stations in Iowa and Indiana show 

 that cross-pollination is absolutely essential to the setting of the seed; 

 that honeybees are very important as pollinators; and that all the 

 mechanical methods of pollination that have been suggested and 

 tested are of no practical value. The increased plantings of sweet 

 clover, due in a large measure to the recommendation of the bureau, 



