EFFECTS OF LIGHT AND TEMPERATURE ON PLANT GROWTH 563 



pink or purple plastic sheets were mounted, and diflFerent intensities 

 were obtained by placing the plants at varying distances from the light 

 source. At low intensities (about 1,000 foot-candles) the unscreened 

 Hght gave most growth, but at high intensities (2,000 foot-candles) 

 both peas and tomatoes grew significantly more under the purple filter 

 than in the unfiltered light. The plants under the pink screen grew 

 poorest (at 14° C. tomatoes grew 4.2 mm/day under the pink filter, 

 4.4 mrn. under no filter, and 5.0 mm. under the purple filter; in the 

 same conditions peas grew 4.8, 5.6, and 5.8 mm., respectively). 



Therefore we can conclude that tlie pink filter did not remove the 

 proper wave length but the purple filter did, and that the inhibitory 

 e£Fect of green light is observable only at high intensities of green light. 



The latter conclusion was confirmed by experiments carried out by 

 Dr. U. Brodfiihrer (unpublished), who found no inhibition of growth 

 or dry-weight production when light from a green fluorescent (i.e., 

 low-intensity ) lamp was added to a mixture of red and blue light. The 

 intensity of the added green light (wave lengths between 5,000 and 

 6,000 A.) did not exceed one quarter of the intensity of the red and 

 blue ( 4,000 to 5,000 and 6,000 to 7,000 A. ) . 



There is no information in the literature that green light inhibits 

 photosynthesis. The experiments of Dr. Dunn in the Earhart Labora- 

 tory (see Went, 1957) also indicate that photosynthesis in tomato 

 plants, when measured as dry-weight production, is low, because of 

 low absorption, but is not inhibited. The results I have cited, there- 

 fore, must be interpreted in terms of the effects of green light on 

 growth. This effect can lead secondarily to inhibition of photosynthesis, 

 as demonstrated for tomatoes ( Went, 1957 ) . 



If growth is inhibited by high-intensity green light, then one may 

 expect to find this effect in nature, where the intensity of the green 

 portion of the sun's rays is high. This may account for the inhibition of 

 stem growth during the day that has been observed by so many in- 

 vestigators since Sachs described it for the first time. This leads to two 

 suggestions : 



1. Installation of purple filters or purple glass in greenhouses 

 might increase the growth of plants by removing the inhibiting green 

 rays. In several instances this is being done by practical growers, ap- 

 parently with good success. 



2. In nature we often find red or purple anthocyanin in the epider- 

 mal cells of growing shoots, especially of plants growing in full sun- 

 light (Vaccinium, Acer, Qucrcus, Sassafras) or in the tropics. The ex- 

 planation given for this phenomenon has usually been that the pigment 

 serves to screen young tissues against too intense radiation or to pre- 

 vent overheating. We may now suggest that it acts as a screen not 



