EFFECTS OF REGIOXS OF VISIBLE SPECTRUM 787 



liydrophytc, mesophyte, and xprophyte, which are based on water rela- 

 tions, a classification based on light relations, namely, polyactinophytes, 

 plants of climates with intense radiation; mesoaktinophytes, plants in 

 climates with medium radiation intensity, and oligoactinophytes, plants 

 in climates having extensive (or more highly diffused) radiation. 



The studies of the various workers mentioned, on reflection, absorp- 

 tion, and transmission of radiation in different parts of the spectrum, 

 should prove of considerable value in interpreting the effect of light on 

 photosynthesis and other physiological processes. It is likewise interest- 

 ing to note that the blue-violet end of the spectrum, which is uniformly 

 absorbed to a greater extent than the red end, also seems to be more 

 effective in producing a plant of normal stature than does the red end. 

 The red end of the spectrum produces a type of etiolation. 



CONCLUDING REMARKS 



Viewing as a whole the work that has been done on the effect of quality 

 of light on plants, we find that the behavior of different species is so varied 

 that it will probably never be possible to generalize. There is, however, 

 sufficient evidence to enable one to say with some certainty that the red 

 end of the spectrum stimulates stem elongation, while the blue-violet 

 end checks it. Best growth has uniformly been obtained in the full 

 spectrum of daylight. No other light source and no filter thus far used 

 has proved superior to daylight as normally received by plants. It may 

 be possible, however, to use a particular quality of radiation for obtaining 

 a specific kind of growth. The investigations thus far carried out have 

 yielded much toward an understanding of the response of plants to radia- 

 tion, but much still remains to be done before we shall understand the 

 mechanism of this response. Photochemical studies carried out in con- 

 nection W'ith growth responses and not restricted to photosynthesis would 

 probably prove enlightening. 



REFERENCES 



1. Arthur, J. M. Red pigment production in apples by means of artificial light 

 sources. Contr. Boyce Thompson Inst. 4: 1-18. 1932. 



2. Arthur, J. M., J. D. Guthrie, and J. M. Newell. Some effects of artificial 

 climates on the growth and chemical composition of plants. Amer. Jour. Bot. 

 17:416-482. 1930. 



3. Bass.\lik, K. (Formation of oxalic acid in green plants.) (Trans, title.) Verh. 

 Schweiz Naturf. Ges. (Jahresversammlung Zurich 1917) 99: 226-7. 1918. {In 

 Chem. Abs. 14: 1841.) 



4. Brown, H. T., and F. Escombe. Researches on some of the physiological 

 processes of green leaves with special reference to the interchange of energy 

 between the leaf and its surroundings. Proc. Roy. Soc. [London] B, 76: 29-111. 

 1905. 



5. BuRGERSTEiN, A. Die Transpiration der Pflanzen. Gustav Fischer; Jena, 1920. 



6. Canals, E. (Effect of different kinds of solar radiation on the formation of 

 essential oils in plants.) (Transl. title.) Bull. Sci. Ind. Roure-Bertrand fils 

 (IV) 3: 8-13. 1921. 



