SOURCES OF HEAT. TRANSFORMATION OF LIGHT INTO HEAT. 



519 



about twenty-nine days before Christiania. And yet the difference of the geo- 

 graphical latitude between Athens and Vienna amounts to 10 13', and that 

 between Vienna and Christiania to 11 43'; from which it would be expected that 

 Vienna would have a start of fifty-one days in advance of Christiania. 



One is tempted to think at first, in explanation of this phenomenon, that 

 growth depends upon the formation of constructive materials from inorganic food; 

 that this latter process can only be accomplished under the influence of light; and 

 that therefore light so far is important for growth. On the other hand, it is 

 difficult to imagine that the light enjoyed by plants growing in Athens should 

 not be sufficient for the formation of organic compounds in the green cells, and 

 for the production of a sufficient quantity of building materials, since, as a matter 

 of fact, the species in question do not appear in any worse condition in Athens 

 than in Christiania, which, however, it must be supposed would be the case if 

 there were a disparity between the food absorbed, metabolism, and growth. This 

 phenomenon suggests rather that the light in the north is able to take the place 

 of heat. And herein lies also the solution of the problem. Not only is there 

 compensation alone; but the light is changed into heat before it acts on the build- 

 ing materials. A portion of the light falling on the plants is reflected, another 

 portion penetrates into the plants, and of these latter rays part bring about the 

 transformation of carbonic acid into carbohydrates, and increases the store of 

 chemical energy, while another portion is changed into heat. This applies par- 

 ticularly to those light-rays which are most vigorously absorbed by chlorophyll 

 and anthocyanin, and which also cause the fluorescence of these colouring-matters; 

 and among the tasks assigned to chlorophyll and anthocyanin, the transformation 

 of light into heat is certainly not the least important. 



But with this we come back once more to anthocyanin that remarkable 

 colouring-matter which has repeatedly been spoken of in detail. It has been 

 mentioned that anthocyanin frequently occurs only on the under side of foliage- 

 leaves. This is observed especially among plants in the depths of shady forests, 

 which, although belonging to widely-differing families, agree in a remarkable 

 manner in this one point. One group of these plants has thick, almost leathery, 

 evergreen leaves lying on the ground, which arise from subterranean tubers, or 

 root-stocks, or from procumbent stems. The widely -distributed Cyclamen 

 europceum may serve as a type of this group. A vertical section of a similar 



