Strawberries produce abundant 

 fruit when the day length is 14 or 

 15 hours, but they usually do 

 not flower when the day length is 

 only 1 or 1 1 hours. 



of plant growth. The shading 

 of certain crops, such as to- 

 bacco and coffee, has become 

 an established field practice, 

 whereas the elimination 

 of shade through removal of 

 trees or other obstructions 

 has been found advanta- 

 geous for various other 

 crops. Despite these gen- 

 eral observations, certain 

 details of plant behavior 

 remained unexplained on 

 the basis of variations in 

 light intensity only, 

 Among these was the fact 

 that in nature many plants 

 bloomed only at definite 

 seasons of the year. Some 

 plants, such as poinsettia, 

 could be propagated by 

 means of cuttings made at 

 various times of the year, 

 but all the new plants, large 

 and small, generally 

 bloomed at approximately 

 the same time. Various 

 economic plants when 

 grown in the North bloomed 

 and set seed but did not do 

 so in the South. Similarly, 

 many kinds grown from 

 seeds were leafy and grew 

 luxuriantly when sown at 

 certain seasons of the year 

 but remained small and 

 went to seed quickly at 

 other seasons. From time 

 to time many explanations 

 for these various behaviors 

 were advanced, but none ad- 

 equately accounted for them. 



