CLIMATIC FACTORS : RADIANT ENERGY 143 



Commercial greenhouses are making use of supplementary light- 

 ing and controlled period of illumination to bring crops into 

 flower on special days or to produce maximum vegetative growth. 

 Growing a crop for its vegetative parts in one latitude for which 

 seeds must be produced in another latitude is now common prac- 

 tice. 



Ecological Significance?— It is thus apparent why many plants 

 in the tropics, where the light period is almost constantly twelve 

 hours, flower throughout the year and, likewise, why so few 

 plants in the United States, even in the South, have this character- 

 istic. It is apparent, too, that arctic species must be long-day plants 

 and why they rarely flower when brought farther south. Also, 

 short-day species could not survive in the tropics since they would 

 not reproduce. Species requiring high temperatures and long days 

 to mature are definitely limited in their northern range. The for- 

 mation of abscission layers in leaves of trees and their decline in 

 physiological activity are initiated in response to shortening days, 

 not to reduced temperature. Therefore, at or beyond the northern 

 limits of their range, trees may be killed by frost because they are 

 not yet sufficiently dormant to withstand low temperatures when 

 they occur. 



It should not be assumed that plant distribution is primarily de- 

 termined by length of day. Many species are little affected by it. 

 Also temperature fluctuations have been shown to modify photo- 

 periodic requirements and responses in several species. Photo- 

 period is just another factor, which may operate with temperature, 

 moisture, and light to determine the range and distribution of a 

 species. 



GENERAL REFERENCES 



H. A. ALLARD. Length of Day in Relation to the Natural and Artificial 



Distribution of Plants. 

 R BURKHOLDER. The Role of Light in the Life of Plants. 

 R. F. DAUBENMIRE. Plants and Environment. New York : John Wiley and 



Sons Co., 1948. 424 pp. 

 W J. Humphreys. Ways of the Weather. 



H. L. SHIRLEY. Light as an Ecological Factor and Its Measurement. 

 U. S. DEPT. Agr. Climate and Man. 

 H. B. Ward and W E. Powers. Weather and Climate. 



