NO. 



14 EFFECTS OF INFRA-RED ON PLANTS— JOHNSTON 



in this paper. With the exception of set i all the individuals of the 

 various groups are very uniform, with but slight variations. For 

 better comparison a representative culture from each group was 

 selected and photographed. These are presented in Plate 4- 



DISCUSSION 



Growth of plants under different wave lengths of light has re- 

 ceived considerable attention at the hands of plant investigators. 

 Much of this earlier work has been reviewed by Teodoresco (18, 19), 

 who has done very valuable work along this line. He investigated 

 two main spectral regions (the blue-violet and the red-orange) by 

 means of colored solutions and glass filters. Because of the general 

 conclusion of many previous investigators and his own experience 

 that infra-red has no appreciable effect in addition to heatmg. he 

 did not think it necessary to use water screens between his light 

 sources and the plants. However, in measuring his light intensities 

 a screen of water and copper acetate was used to eliminate the effect 

 of the infra-red upon his measurements. His general conclusions 

 from experiments with a large number of land plants, many of which 

 were duplicated with both glass and solution filters, are: that in 

 the longer wave lengths, internodes were elongated and more numer- 

 ous areas of leaves reduced, the leaves themselves were thinner, 

 and a general abnormal configuration resulted. In the shorter wave 

 lengths growth in length was retarded, leaf area increased as well as 

 leaf thickness. The general appearance of the plants was normal, 

 resembling those grown in white light. In many respects the plants 

 grown in the red-orange region resembled plants grown in darkness 

 except for their green color. As Teodoresco points out, plants grown 

 in darkness became etiolated without chlorophyll assimilation, so that 

 growth ceases. In red light, however, the plants grow for a longer time 

 than in darkness, due to the production of food by photosynthesis. 

 In view of the fact that different color filters usually transmit dif- 

 ferent amounts of infra-red light, the existence of an effect of radia- 

 tion in the near infra-red would necessarily qualify the conclusions 

 to be drawn from such experiments. Consequently, the results ot 

 the present investigation raise serious doubts as to the validity of 

 this type of experiment, unless it is definitely shown that the different 

 filters transmit the near infra-red in the same degree. 



Funke (7), working mainly with aquatic plants, studied the effects 

 of three general spectral regions, red, green, blue, and subdued white 

 light Any ultra-violet or infra-red passing through his filters was 



