In the present emergency and the years ahead, American farmers will 

 be called upon to produce many crops that are new to them. New crops 

 inevitably bring new problems of production. Much waste of effort and 

 invested capital may often be prevented by knowing the areas to which 

 these new crops are adapted. 



There are three important environmental factors affecting the adaptability 

 of plants to a given area — soil, climate, and length of day. In agricultural 

 research and in farm practice much attention has been given to the soil and 

 climatic requirements of plants, but less attention has been paid to the day- 

 length requirements. Nevertheless, existing knowledge in this field is now 

 being used in crop production and breeding and results of research have 

 helped to eliminate crop failures and have guided farm practices throughout 

 the world. 



This publication ' summarizes many of the agricultural applications that 

 have resulted from studies of the influence of day length on plant develop- 

 ment and calls attention to the effect of day length on such strategic crops 

 as soybeans, sugar beets, and hemp. 



1 The information and illustrations given herein were contributed by a number of individuals and are 

 based on experiments carried out in several divisions of the Bureau of Plant Industry. The publication 

 was prepared under an allotment from the special Research Fund authorized by Title I of the Bankhead- 

 Jones Act of June 29, 1935. 



Washington, D. 0. Issued September 1942 



