NO. 2 PLANT GROWTH AND WAVE-LENGTH BALANCE — JOHNSTON 1 5 



dioxide is the limiting factor, the dry weight increases with increased 

 illumination. Hoover, Johnston, and Brackett (1933), working with 

 wheat plants, found that in normal air CO2 became limiting at a light 

 intensity of about 0.05 to 0.06 watts/cm\ In none of these experi- 

 ments with the tomato plant was the intensity greater than these 

 values. Although the two plants may not behave exactly alike, it is 

 reasonable to suppose they are similar enough to assume that at no 

 time was CO2 the limiting growth factor. In order to accentuate 

 growth differences due to wave-length mixtures and minimize the 

 effect of intensity on dry weight production, the dry weight data 

 were divided by watts/cm^ This dry weight efficiency of comparable 

 cultures in the last three experiments was used as a criterion of the 

 effect short-wave (blue) radiation added to that of longer wave length 

 had on plant growth. It would appear from the data given in table 

 12 that a greater amount of dry weight is produced with a Mazda 

 light by enriching it with blue from a mercury lamp to the extent of 

 14 to 51 percent under the conditions of these experiments. Care 

 should be exercised in drawing any far-reaching conclusions, for with 

 a change in quality or wave-length distribution of the Mazda or other 

 source rich in red, changes undoubtedly will be necessary in other 

 portions of the spectrum. Although for good growth plants very 

 probably tolerate a rather wide range in wave-length distribution, yet 

 it would appear that the more nearly this distribution in artificial 

 light approaches that of sunlight the better will the plants grow. 



SUMMARY 



Emphasis is placed on the importance of quality or wave-length 

 distribution of light in affecting plant growth. A method and several 

 experiments are described in which plants were grown in " mixed " 

 lights. By placing the plants on small rotating tables between two 

 light sources, one rich in red, the other rich in blue, the proportion 

 of each type of radiation falling on each culture was varied by the 

 position of the culture with reference to the light sources. 



As found in previous experiments, yellowing of leaves occurred 

 in light rich in near infrared. Since this trouble could be corrected 

 to a considerable extent by the type of nutrient solution used, it 

 indicates the importance of wave-length distribution on the uptake 

 of mineral nutrients. 



Excellent growth under entirely artificial conditions was obtained 

 with plants grown between a 1,000-watt, water-jacketed, projection 

 Mazda lamp and a 400-watt, high-pressure mercury lamp. The posi- 



