ASSIMILATION AND LIGHT. 



301 



in the seed. That the greening is for and by itself no assimilation process, however, 

 and that it takes place without the decomposition of carbon dioxide, may be de- 

 monstrated further with the aid of the apparatus here figured. The yellow leaves, 

 previously grown in the dark, are placed under the bell-glass, and, when the apparatus 

 stands in the light, become green, even if the plate is filled with a strong solution 

 of potash which completely absorbs the carbon dioxide under the bell-jar. 



Finally is to be mentioned the fact that the organ of assimilation, even when 

 it has been previously normally developed in the light, can dispense with the 

 light for a time (in the normal course of things it does so every night) without 

 taking any harm thereby; but more permanent darkness, or even only deep 

 shading (i. e. for several days or weeks), brings about a disease of the previously 

 green cells, their chlorophyll-grains become destroyed, and the leaves turn 

 yellow and finally perish. This even occurs in the feeble illumination in 

 which, as mentioned above, the same leaves previously became green. Never- 

 theless there are plants, the green organs of which can dispense with light, even 

 for months together, without dying. I found this to be the case for instance with 

 some species of Cactus and Selaginella. 

 In the main, the dependence upon light 

 here referred to is particularly true for 

 quickly growing summer-plants. 



All the relations between the organs of 

 assimilation and light here mentioned, as 

 well as the independence of growth with 

 regard to the latter, must be carefully ob- 

 served, if the important fact that the decom- 

 position of carbon dioxide and assimilation 

 in the chlorophyll are a function of light is 

 to be properly understood. It follows 



from what has been said, that not light of '^^' ^'^' 



any haphazard intensity will do what is 



necessary. Unfortunately we lack photometric methods to enable us to distinguish 

 those intensities of light which come into consideration in assimilation with the same 

 precision, and generally intelligible exactness, as is possible with the thermometer 

 with respect to temperature. The most exact photometric methods, and especially 

 the method proposed by Bunsen, for instance, only give us information as to the 

 intensity of the strongly refractive rays of light, the so-called chemical rays; but 

 these, as I shall show subsequently, only come incidentally into consideration. 

 We must therefore adopt entirely different methods with respect to statements 

 concerning the intensity of light necessary for assimilation, and which will not be 

 here given in detail. Only so much is obvious, that, for the decision of certain 

 questions, use may be made of the law that with double the distance of a 

 surface from a luminous point, the intensity of illumination of the latter sinks 

 to I ; at three times the distance to \, &c.; and that the intensity of illumination 

 of a leaf-surface at the same time depends upon the sine of the angle of incidence. 

 It would cost us too much time, and would moreover lead to no satisfactory result 

 in the end, to enter more in detail into these matters. It must therefore suffice, that 



