

JOINAL OF AGEKDITIAL ISEARCH 



Vol. XVIII Washington, D. C, March i, 1920 No. ii 



EFFECT OF THE RELATIVE LENGTH OF DAY AND 

 NIGHT AND OTHER FACTORS OF THE ENVIRON- 

 MENT ON GROWTH AND REPRODUCTION IN 

 PLANTS 



By W. W. Garner, Physiologist in Charge, and H. A. Allard, Physiologist, Tobacco 

 and Plant Nutrition Investigations, Bureau of Plant Industry, United States Depart- 

 ment of Agriculture ^ 



INTRODUCTION 



The importance of the relationships existing between light and plant ' 

 growth and development has been so long recognized and these relation- 

 ships have been of so much interest to investigators that a very extensive 

 literature on the subject has been developed. For present purposes it 

 will not be necessary to attempt even a brief review of this literature, 

 and only some of the leading features bearing upon the particular prob- 

 lems in hand need to be touched upon. For more extended discussions 

 of the work in this field the monographs of MacDougal {iSy and Wiesner 

 (26) may be consulted. Three primary factors enter into the action of 

 light upon plants — namely, (i) the intensity of the light, (2) the quality, 

 that is, the wave length of the radiation, and (3) the duration of the 

 exposure. Most phases of these three factors have been more or less 

 extensively investigated. In the present investigation we are concerned 

 chiefly with the general growth and development of plants and the 

 reproductive processes as affected by the daily duration of the light 

 exposure. 



As regards intensity, it seems to be pretty well established that there 

 is an optimum for growth in each species and that for many species this 

 optimum is less than the intensity of the full sunlight on a clear day. 

 Within limits, reduction in light intensity tends to lengthen the main 



1 The authors desire to acknowledge their indebtedness to Prof. C. V. Piper, in charge of Forage Crop 

 Investigations, Bureau of Plant Industry, for helpful suggestions, to Mr. W. J. Morse, of the Office of 

 Forage Crop Investigations, Bureau of Plant Industry, for seed of certain varieties of soybeans and infor- 

 mation as to the characteristics of these varieties, to Dr. D. N. Shoemaker, of the Office of Horticultural 

 and Pomological Investigations, Bureau of Plant Industry, for similar assistance as to certain varieties 

 of ordinary beans, and to Prof. H. H. Kimball, of the Weather Bureau, United States Department of 

 Agriculture, for important data relating to the shading effects of nettings of different mesh used in these 

 investigations. 



* Reference is made by number (italic) to "Literature cited," p. 605-606. 



Journal of Agricultural Research, Vol. XVIII, No. ix 



Washington, D. C. Mar. 1, 1920 



KeyNo.G-i86 



-'-' (553) 



