INFLUENCE OF THE DIFFERENT RAYS OF THE SPECTRUM 347 



value, it is impossible that the assimilatory curve can precisely correspond 

 to the distribution of energy in the spectrum (Fig. 52, En.), or to any such 

 physical curve. The brightness of the light to the sensitive retina of 

 the eye does not form any safe test of its photosynthetic activity; 

 and although a general relationship exists between the optical intensity 

 of light and its assimilatory power 1 J it is not the brightness of the light, 

 but the amount of energy that can be obtained from it, which is of primary 

 importance. 



The energy for the decomposition of carbon dioxide in green chloro- 

 plastids is obtained from the rays having a wave-length of from 391 /^u 

 (ultra-violet) to 770 /x/x (infra-red), and a vibratory activity of 800 to 400 

 billion (io ia ) times per second, whereas in the purple bacteria it is the 

 infra-red rays of 900 to 800 jujx wave-length which are most active. In 

 green plants the most active decomposition is caused by the rays of 

 660-680 W wave-length, and further towards the red end of the spectrum 

 the curve falls steadily (Fig. 51). A similar relationship exists for photo- 

 chemical actions on dead substances ; moreover, most physiological functions 

 exhibit minimal, optimal, and maximal points, while secondary maxima may 

 be shown similar to those which the assimilatory curve exhibits 2 . 



A precise determination of the course of the assimilatory curve has 

 not as yet been obtained, for even by the delicate bacterium method, 

 assimilation is only rendered apparent by the evolution of oxygen, i.e. 

 when the respiration is overpowered, and the same objection and others also 

 apply to the bubble-counting method. It is only when respiration can be 

 estimated that the actual amount of assimilation may be calculated, and 

 an evolution of oxygen becomes perceptible at the extreme ends of the 

 spectrum only under light of great intensity. Since the rays of shorter 

 wave-length undergo more marked dispersion than the red rays do, the 

 blue end of the spectrum is spread over a- larger area, and since the light 

 is correspondingly diluted, the evolution of oxygen ceases to be perceptible 

 sooner at this end of the spectrum than it would be if this were not the case. 

 This error has been allowed for in constructing the curves in Figs. 51 and 52. 

 Just as different animals and even different people have not the same 

 range of colour perception, so the assimilatory curve for different green 

 plants may not always end at precisely the same region of the spectrum. 

 Bonnier and Mangin 3 have observed that in certain cases a slight assimila- 

 tion of carbon dioxide occurs in the ultra-violet rays, though not sufficient 

 to overpower respiration, whereas Pfeffer found that behind a solution 



1 Cf. Pfeffer, Bot. Zeitung, 1871, p. 319; Sachs, Arb. d. Bot. Inst. in Wurzburg, 1872, Bd. I, 

 p. 276. 



a It is easy to show, by means of the bacterium method, that yellow sodium light can induce 

 active photosynthesis, although Beyerinck supposes that it is inactive (Bot. Zeitung, 1890, p. 743). 



8 Bonnier et Mangin, Compt. rend., 1886, T. cil, p. 123. 



