NO. 2 PLANT GROWTH AND WAVE-LENGTH BALANCE — JOHNSTON 7 



The near infrared radiation was apparently destructive to chlorophyll 

 or inhibited its formation. This again appeared to be the case for the 

 three plants in culture i. However, one of the most interesting ob- 

 servations made in this experiment was that the color of the plants in 

 culture 2, which received the same radiation intensity as those of 

 no. I, was much greener. This color difference is seen to some extent 

 in plate 2 as differences in light and dark tones of the plants in the 

 upper and lower figures. This was also true for the plants in cul- 

 tures 4 and 6, as compared with cultures 3 and 5 respectively, which 

 were grown under similar light conditions. All the plants grown in 

 nutrient solution to which (NH4)2S04 had been added were greener 

 than the corresponding ones without this additional nitrogen. This 

 observation suggests the influence of the type of radiation on the up- 

 take of mineral nutrients. This same solution without the (NH4)2S04 

 has been used in growing tomato plants in the greenhouse but the 

 characteristic chlorotic effects were not noted until the plants were 

 grown under Mazda lamps. 



The percentages of added mercury radiation to total illumination 

 were 3, 15, and 50 respectively for cultures 1-2, 3-4, 5-6. The green 

 color of the leaves was deeper where this percentage was larger. A 

 more striking color difference occurred, however, between the plants 

 in cultures with and without the (NH4)2S04. 



The average total dry weight per plant for each of the three light 

 conditions 1-2, 3-4, 5-6 was .757, .549, and .443 gram respectively. 

 Under these three light conditions the efficiency in the production of 

 dry weight per watt/cm" was 18, 27, and 34 respectively. Although 

 the total intensity of 5-6 was about a third that of 1-2, on the basis 

 of efficiency in producing dry weight per unit energy, the less intense 

 radiation was about double that of the more intense. 



One other factor in addition to wave-length distribution must be 

 recognized in an experiment of this type. One lamp (Mazda) gave 

 practically continuous illumination; the other (mercury, 60-cycle), a 

 fluctuating illumination varying from a minimum considerably below 

 the average to a maximum much greater than the average as de- 

 termined by the thermocouple and photoelectric cell. McAlister 

 (1937) clearly shows that a change in efficiency of carbon dioxide 

 assimilation occurs with frequency of intermittency of illumination. 

 Although it may be comparatively safe to compare the different cul- 

 tures in any one experiment since the " flicker " effect is doubtless 

 the same, it is impossible to compare results of experiments in which 

 the light is continuous with those in which it is intermittent or with 

 those in which it is half continuous and half intermittent. 



