6oo FACTORS AFFECTING GROWTH 



lead to the not surprising general conclusion that the full spectrum of sunlight 

 is more satisfactory for the development of plants than any spectral portion 

 thereof, or than any known artificial source of illumination. 



As a rule plants grown under the longer wave lengths of the visible 

 spectrum resemble etiolated plants more or less closely. Internodes and 

 petioles elongate more than in white or blue-violet light, and the leaves fail to 

 expand fully (Popp and Brown, 1936 a). 



Plants allowed to develop in a blue-violet light are usually similar in 

 gross morphology' to those allowed to develop in "white light," although 

 they are often smaller and more compact. Petioles and internodes are 

 shorter than when plants develop in red light, and leaves expand in an approxi- 

 mately normal fashion. Apparently the development of their usual configura- 

 tion by plants is largely due to an effect of the shorter wave lengths of the 

 visible spectrum, since when these wave lengths are missing from the incident 

 light disproportionate development of the organs occurs as previously described. 



Recent experiments of Johnston (1932), however, show that tomato plants 

 exposed to a relatively high intensity of infrared radiation have longer inter- 

 nodes, larger leaves, and less chlorophyll than similar plants receiving the 

 same quantity of visible radiation, but no infrared. The influence of infrared 

 radiation upon plants results at least in part and perhaps entirely from its 

 heating effects. 



All wave lengths of ultraviolet shorter than those found in the sunlight 

 which impinges upon the earth's surface have a retarding effect on growth 

 and often a destructive influence on plants (Popp and Brown, 1936 b). The 

 effects of the sunlight ultraviolet which naturally falls upon plants are, in 

 general, very similar to those of the blue-violet region of the visible spectrum. 



The majority of numerous investigations indicate that X-rays exert only 

 destructive effects upon plants, although a few workers are of the opinion 

 that very weak doses sometimes have a stimulatory effect (Johnson, 1936). 



The exact influence of different light qualities can only be completely 

 evaluated by testing the effects upon plants of exposure to relatively narrow 

 bands of light of equal intensity in various parts of the spectrum. No critical 

 experiment of this sort has ever been performed upon any of the higher plants. 

 Meier (1936), however, exposed cultures of an unicellular alga {Stichococcus 

 bacillaris) to full daylight, to infrared radiation, and to five narrow ranges of 

 wave lengths under artificial light. The radiation provided was equal in 

 intensity for all cultures except those exposed to daylight. Growth was 

 measured in terms of cell multiplication during a two weeks exposure. In- 

 crease in the number of cells in the daylight cultures was more than fourfold, 

 in blue light (400-520 W)it) over threefold, and in both red (600-750 7«/x) 



