NO. 2 PLANT GROWTH AND WAVE-LENGTH BALANCE JOHNSTON I3 



such plants as begonia, gardenia, cotton, geranium, buckwheat, and 

 snapdragon. Although this light source was not satisfactory for the 

 tomato plant, the authors point out that other wave bands of light 

 may be found which should be added or subtracted for the best 

 growth of some plants such as the tomato. 



Dastur and Mehta (1935) determined the rate of photosynthesis in 

 approximately equal intensities of red, blue, and white light. Photo- 

 synthetic activity was greatest in the white light, intermediate in the 

 red light, and least in the blue light. They state that both the red and 

 blue regions are necessary for normal photosynthesis. 



Equally interesting are the results of Hoover's (1937) investigation 

 on determining the rate of CO2 absorption as a function of wave 

 length on the basis of equal incident energy. The principal maximum 

 occurred at 6500 A in the red, and a secondary maximum came at 

 4400 A in the blue. The greater transmission and reflection of radia- 

 tion in the green region decreased the effectiveness in that portion 

 of the spectrum. The limits of CO2 absorption were placed between 

 7200 A and 7500 A in the red, and below 3650 A in the blue end of 

 the spectrum. 



Dastur and Solomon (1937) show the importance of the blue- 

 violet end of the spectrum in photosynthesis in a series of experi- 

 ments in which plants are grown in the light of a carbon arc, in 

 " mixed " light where the gas-filled electric lamp light has super- 

 imposed upon it a beam of blue-violet light, and in the light of the 

 gas-filled electric lamp alone. The " mixed " light was composed of 

 two beams originating in a single source (1,000-watt flood lamp) and 

 reflected to the plant by mirrors. One beam was passed through a 

 copi^er sulphate filter which limited the wave-length band to the 

 region 4200 A to 4720 A. These beams (white and blue) were 

 reunited in the proportion 1:1 on an intensity basis. Plants grown 

 in these three lights showed greatest photosynthetic activity in the 

 carbon arc light, intermediate in the " mixed " light, and least in the 

 gas-filled electric bulb light. This follows the order of richness in 

 blue-violet light of the three sources. 



From the foregoing discussion it would appear that plants can be 

 grown in artificial light, but for more or less normal growth the light 

 should include those wave lengths found in the visible solar spectrum. 

 An increase in intensity or the absence of a given portion of this 

 spectrum brings about abnormal growth responses. Undoubtedly, 

 the more nearly the artificial light resembles sunlight in its energy 

 distribution, the more nearly normal are the plant growth responses. 



