Day Length and Crop Yields 



By M. W. PARKER, physiologist, and H. A. BORTHWICK, morphologist, 

 Bureau of Plant Industry, Agricultural Research Administration 



The successful production of crop plants is dependent upon manj 

 factors, such as relative fertility of the soil, rainfall, soil moisture, tem- 

 peratures prevailing ai different tunc- of the year, and light condi- 

 tions. It is seldom that all these factors are present in any given 

 locality in such combination that it is not possible for man to modify 

 them to the extent that more abundant crop yields of better quality 

 are secured. Indeed, such modification of the environment in which 

 plants grow and the selection of plants adapted to specific environ- 

 ments constitute a large part of agricultural practice. It is the special 

 duty of the experimentalist to find ways and means of accomplishing 

 these desired results in the most direct and economical manner possi 

 Ble. The search for and the substitution of more effective practices 

 for those which are less efficient are the marks of progress of an ever- 

 changing agricull ure. 



It has long been known that light conditions affect plants in a variety 

 of ways. Many studies have shown, for example, that the intensity 

 of light is an important factor- in determining the character and rate 



Timothy, a hay crop, heads during the long days of early summer. 



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