EFFECT OF RADIANT ENERGY ON GROWTH 599 



directly with the intensity of the h'ght, while the best general development of 

 leaves, flowers, and fruits occurred in the greatest light intensity used, which 

 was approximately half the intensity of the noon sun on a clear summer's day 

 (Chap. XIX). 



Somewhat similar results were obtained by Shirley (1929, I936) who 

 studied the efiects of differences in light intensity upon the development of a 

 number of species. In general the absolute dry weight, percentage of dry 

 matter in the tops, rigidity of the stem, and leaf thickness all increased with 

 increase in the intensity of light up to full sunlight, providing other factors 

 were not limiting growth. Maximum height of the plants and maximum 

 leaf area were attained, on the other hand, at light intensities considerably 

 below that of full summer sunlight. Low light intensities also resulted in a 

 considerable delay in the time of maximum flowering and fruiting. 



In general the results of these and other similar investigations indicate 

 that maximum height and leaf area are attained at light intensities which are 

 considerably less than full summer sunlight. Relatively high light intensities 

 result in most species in shorter internodes, plants of lower stature, and smaller 

 leaves, but the dry weight, number and size of the branches, size of the root 

 system, and production of flowers and fruits is greater than in weaker light 

 intensities. Many species show increased growth in terms of dry weight 

 increment with increased light intensity up to lOO per cent of summer sun- 

 light, if no other factor is limiting. All phases of the growth of typical shade 

 species are usually retarded, however, by high light intensities. 



2. Quality. — Because of the experimental difficulties involved no entirely 

 critical study of the exact effects of different light qualities upon the develop- 

 ment of the higher plants has ever been carried out. Various qualities of light 

 for experimental work on plants are usually obtained by allowing sunlight or 

 artificial light to pass through filters of colored glass, gelatin, or other ma- 

 terials. If the energ}' spectrum of the light source and the transmissive prop- 

 erties of the filter are known the quality of light falling upon the plants placed 

 under such filters can be determined. It is difficult, however, to select or 

 adjust filters so that the different qualities of light used are all equal in inten- 

 sit}'. Furthermore most qualitatively different light sources which can be 

 experimentally produced either by means of filters or in other ways are of 

 relatively low intensity, which necessarily limits the comprehensiveness of 

 studies which can be made of this problem. For these reasons most conclusions 

 regarding the influence of different light qualities upon growth in plants are 

 of restricted application. 



All experiments upon the effect of limited ranges of wave lengths of light 



