EFFECTS OF VARIOUS COLOURS OF LIGHT. 



305 



hydrochloric acid. The action of the red-yellow light on the decomposition of 

 carbon dioxide which we have established contradicts no fact, but only a false 

 generalisation; since it shows that other chemical processes which take place in 

 the chlorophyll are brought about by other rays of light, namely the red-yellow. 

 As regards the method of experimenting, I subsequently devised a more con- 

 venient arrangement. Glass vessels with parallel walls, and as large as possible 

 (so-called Cuvettes), were filled with the solutions, and fixed somewhat like windows 

 into opaque boxes ; in these, by means of a door at the back, the plant to be 

 observed is placed in water containing carbon dioxide. 



So long ago as 1844 Draper had observed the evolution of oxygen from green 

 leaves, by placing them, enclosed in small glass vessels, in various regions of the 

 solar spectrum, and determining the quantities of oxygen given off, and thus 

 learning the effects, of the different coloured parts of the solar light. He found 



FIC. 221.— The curves of assimilation, temperature, brightness, a 

 are represented on the spectrum of chlorophyll (AH) as an abscissa. 



that the effect in the red portion is extremely feeble, rising quickly in the red- 

 orange, and reaching a maximum in the yellow-green ; it falls again in the blue 

 of the spectrum to an extremely small quantity. This obviously agrees with the 

 researches described above, where the light passed through coloured solutions. 

 If we suppose the solar spectrum to be an abscissa line, on which the effects 

 discovered by Draper are erected as ordinates, the relative quantities of oxygen 

 evolved in equal times in the various regions of the spectrum appear in the 

 form of a curve, which begins in the red, reaches its maximum in the yellow- 

 green, and then quickly sinks again towards the blue. This Draper's curve, 

 as we may term the law of the dependence of assimilation upon the colour of 

 the light,, was tested more exactly by Pfeffer in 1870 by means of the eudio- 

 meter, and then, in 1872, by counting the bubbles, a method employed by me 

 previously \ The result of his observations may be understood most simply from 



I 



' Investigations on assimilation in coloured light are collected in Pfeffer's ' Pflanzen- Ptiysiologie, 



l,p. 211. 



[3] 



