GRAPHIC REPRESENTATION OF EFFECTS OF LIGHT, HEAT, ETC. I95 



curves : i. e., they begin at a certain intensity of the external influence, and" thus 

 at a definite place on the abscissa line, then ascend more and more until 

 a maximum of the effect appears above a given point of the abscissa line, which 

 is always to be distinguished as the optimum point, from whence onwards the 

 curves again sink down to the abscissa. In order to illustrate this generalisation 

 of our law of curves more clearly by an example, I may cite the dependence 

 of the evolution of oxygen from the organs containing chlorophyll, under the 

 influence of light of various colours ; for the evolution of oxygen effected by 

 the chlorophyll is a function of the wave-lengths of light, in so far that only light 

 the wave-length of which amounts to at least 0-0003968 mm., and does not 

 exceed 0-0006866 mm., effects the separation of the oxygen. Starting from both 

 extremes, this effect of the light increases, and reaches the maximum at an optimum. 



A ssimilatioii. 

 Brightness. 

 Heat. 

 Chemical action. 



Red. Ormige Yellcnv. Green. Blue. 



Indigo. Violet. 



FIG. 188.— The band A—H denotes the absorption spectrum of a solution of chlorophyll. The lines j4, R—H are 

 Fraunhofer's lines. The spectrum is employed as the abscissa-line for the curves explained in the figure (after 

 Pfeffer). 



wave-length of 0-0005889 mm. As is well known, the various wave-lengths of 

 light produce in our eyes the sensation of the various colours of the spectrum ; and 

 since the number first mentioned represents the wave-length of the blue, and the 

 second that of the red, while the third — the optimum — denotes the wave-length 

 of yellow light, we may also say that the evolution of oxygen begins in the blue 

 light, ascends thence through the green of the spectrum up to the middle of the 

 yellow, and there reaching its highest point again descends in the orange-coloured 

 rays, to cease within the red portion of the spectrum. Thus if we have a sufficiently 

 large solar spectrum, and a green leaf is placed during equal times in the various 

 coloured regions of the spectrum, the quantity of oxygen evolved each time is 

 expressed by the curve of which the cardinal points were specified above^. 



' With reference to the effect of various coloured lights on assimilation, cp. my treatise m ' Bot. 

 Zeit.' 1864, pp. 353, &c., where the older literature, up to that time scarcely noticed at all, is 



2 



