22 PHYSIOLOGY OF NUTRITION 



§5. Influence of Light upon the Decomposition of Carbonic Acid by Plants. 



An acquaintance with the properties of the different rays of the sun's spectrum 

 (Fig. 12) is prerequisite to an understanding of the researches devoted to this 

 subject. Only the central part of the spectrum, approximately that portion 

 lying between lines A and H, is visible to the human eye; on either side are in- 

 visible rays, infra-red to the left and ultra-violet to the right. Of the visible 

 rays, the yellow are the brightest, the brightness reaching a maximum at line 

 D and decreasing to zero beyond A and H. Brightness does not, however, 

 represent the character of the rays, but only that of the human eye. The en- 

 ergy maximum in the prismatic solar spectrum is usually shown as falling in the 

 region of the infra-red, as in Fig. 12. Nevertheless, recent work upon the dis- 

 tribution of heat in the ordinary diffraction spectrum of sunlight shows the 



Fig. 12. — Graphs of the prismatic solar spectrum. PA, infra-red; AH, visible; HS, 

 ultra-violet rays; PTS, temperature curve; ALH, curve of light intensity; DKS, curve of effect 

 of light upon the decomposition of silver salts. 



energy maximum to lie between lines B and C; 1 and, according* to the latest 

 researches, the position of this maximum is not constant but varies from the 



blue or purple color and frequently masking the green of the chlorophyll in leaves. They are 



red when acid and blue when alkaline. The color of red apples and many other fruits, of 



many red, blue and purple flowers, of the beet-root, of red cabbage, of young leaves of many 



plants, and of the bronze-colored leaves of the copper beech, are due to the presence of these 



pigments. They are often present along with chlorophyll, as in the case of red cabbage and the 



copper beech, and still other pigments frequently accompany them. They are soluble in water, 



alcohol and ether, and the color of the solution alters from red to purple or blue as the reaction 



is altered from acid to neutral or alkaline. For further information see: Haas and Hill, 1921. 



[See note 3, p. 6.] West, Clarence J., Plant pigments: The chemistry of plant pigments other 



than chlorophyll. Biochem. bull. 4: 151-160. 1915. — Ed. 



1 Langley, [S. P.], Observations du spectre solaire. Compt. rend. Paris 95: 482-487. 1882. Idem, 

 Energy and vision. Phil. mag. V, 27: 1-23. 1889. [Sunlight as it reaches plants is so variable in both 

 quality and intensity that each quantitative experiment on photosynthesis, etc., in natural illumination, 

 should be carried out with very careful measurements of solar radiation. Nutting states that the sun's 

 total radiation varies over a range of 8 per cent, of the mean, while the earth's atmosphere, even with a clear 

 sky, absorbs from 20 to so per cent., and this varies from minute to minute and from hour to hour of the 

 day. Nutting gives a table (p. 202) of mean solar energy quantities reaching the surface of the earth at 

 Washington at noon, for 26 different wave-lengths, from 38s to 428/1*4- (See Nutting, P. G., Outlines of 

 applied optics. Philadelphia, 191 2.) The wave-length showing the maximum energy value also varies 

 markedly in natural sunlight. For further information see: Abbot, C. G., and Fowle, F. E., Jr., Primary 

 standard pyrheliometer. Ann. Astrophys. Observ. Smithsonian Inst. 2: 39-47. 1908. Idem, The 

 value of the solar constant of radiation. Astrophys. jour. 33: 191-196. 191 1. Also see Pulling, H. E., 

 Sun-light and its measurement. Plant World 22 : 151-171. 187-209. 1919. — Ed. 



