BUREAU OF PLANT INDUSTRY. 161 



tions (not Farmers' Bulletins, which are reprinted automatically) 

 were reprinted during the year, the total number of copies issued 

 being 18,200. 



For bulletins covering practicalh^ every phase of plant production 

 in the United States, the Chief of the Division of Publications should 

 be addressed. 



PLANT-NUTRITION INVESTIGATIONS. 



The chief feature of the work during the year has been the dis- 

 covery that the seasonal length of day, that is, the duration of the 

 daily light period, is a factor of first importance in determining 

 jDlant develoi^ment. As a result of these studies it has been possible 

 to establish the relationship of the length of day to early and late 

 maturing varieties of crop plants, the relative development of the 

 vegetative and fruiting portions of the plant, the distribution of the 

 growing and the fruiting periods through the year, the condition of 

 " everblooming " or " everbearing," the adaptation of different va- 

 rieties and species of crop plants to different latitudes, and the 

 natural distribution of plant species. It has been demonstrated that 

 the vegetative and the reproductive or flowering and fruiting periods 

 can be suppressed or brought into expression almost at will through 

 artificial control of the length of the daily illumination period and 

 that for this purpose electric light may be successfully used as a sup- 

 plement to sunlight during the short winter days. 



The plant can not attain sexual reproduction, it has been shown, 

 except when it is exposed to a favorable length of day. The require- 

 ments, however, differ widely with species and varieties. Further- 

 more, a length of day that is unfavorable to reproduction may be 

 favorable to growth; under that condition the plant continues its 

 vegetative development profusel}^ and indefinitely without bearing 

 fruit. A length of day may be found that is favorable both to sexual 

 reproduction and vegetative growth, which tends to bring about the 

 " everbearing " type of fruiting. 



This new principle undoubtedly explains the erratic behavior 

 which has been observed with many crops when they are shifted to 

 different latitudes and may also clear up the conflicting results of 

 variety tests and field tests conducted with the same crops but in 

 different regions. 



AGRONOMY, CROP BREEDING, AND HORTICULTURE. 



COTTON. 



Increased production of Lone Star cotton. — The popularity of the 

 Lone Star, an early-maturing variety of the Texas big-boll type, 

 originated by the Bureau of Plant Industry, is becoming greater 

 each year. An important part of the Texas crop is Lone Star cotton, 

 and tlie growth of the variety is being extended in Texas and other 

 States, including southern California, as rapidly as the seed supplies 

 can be increased. In spite of extremely unfavorable conditions of wet 

 weather that injured much of the seed in the fall of 1919 and com- 

 pelled extensive and repeated replanting in the spring of 1920, it 

 was possible, by bringing a stock of pure Lone Star seed from 

 southern Texas, to replant the fields of the Lone Star community 



