560 PLANT GROWTH AND PLANT COMMUNITIES 



cally influences growth in length, whereas blue light causes phototro- 

 pism in the same organ. Although phototropism is also a manifestation 

 of growth, as pointed out by Blaauw, there is no over-all change in 

 growth in the typical phototropic curvature. 



4. There are some cases in which reactions in plants are brought 

 about by a cytochrome absorption. In this case also, very low light in- 

 tensities are effective and the process is soon saturated. 



5. Plants can be grown in light of very limited spectral composi- 

 tion. For instance, when tomato plants are grown in red light only, 

 they develop into long spindly plants which seem more or less etiolated. 

 Leaf growth is decreased, whereas stem growth is greater than in white 

 light. To obtain sufficient growth, high intensities of red light are re- 

 quired. Evidently the plant has a rather high concentration of red-ab- 

 sorbing pigment. If plants are grown in blue light, the stem growth is 

 less than in white light, but the leaf development is normal. Again, 

 high intensities are required to produce this effect. Since the effects of 

 red and blue light are entirely different, it is impossible to attribute 

 these effects to chlorophyll absorption. We must be dealing with two 

 different light-absorbing systems— one absorbing in the blue and the 

 other absorbing in the red. The normal growth obtained when plants 

 are grown in white light must, therefore, be due to the combined effect 

 of the red and tlie blue pigment. 



6. There is an indication that green light, applied at very high 

 intensities, is inhibitory to growth. Of the wave lengths in visible light, 

 green rays are the least absorbed by chlorophyll: consequently green 

 light, per number of quanta falling on the plant, produces less photo- 

 synthesis than does red or blue. This has been shown repeatedly. 



When a plant is grown in green light, it is much lighter in weight 

 than plants grown in equal intensities of red, blue, or white light ( ex- 

 pressed in quanta or in ergs supplied ) , and its growth is retarded, even 

 when high intensities are provided (see, e.g., Went, 1957, Figure 65). 

 Since in this same graph it appeared that plants grown in a combina- 

 tion of red and blue light grew larger and heavier than those grown in 

 white light, the conclusion was drawn that green light was inhibitory 

 to growth. 



A few experiments were carried out to test this conclusion. In a 

 greenhouse, kept at 26° C. during the day and 20° C. during the night, 

 frames covered with colored plastic sheets were placed in a horizontal 

 position over benches, and tomato plants were kept under them in such 

 a way that ventilation was not impaired but that most of the light 

 reaching the plants passed through the filters. (The filters used were 

 all obtained from the Bates Lighting Company and Scenic Study. Each 

 filter is designated by a number: No. 112, flesh pink, absorbs mainly in 

 the region from 4,800 to 5,300 Angstroms; No. 18, deep flesh pink— 



