7() DK. V. MAKTIUS 



fact. They go baud in hand in their inlluences on bodies ; the 

 more lifrbt these receive and retain, the greater is the heat pro- 

 duced within them ; and as soon as tliey exceed in degree that of 

 surrounding bodies, heat, not light, is radiated from them. The 

 relation in which plants stand, as regards those powers, corresponds 

 in some respects with that which e.xists in lifeless bodies ; and in 

 others it differs, and even in their different stages, as is instanced 

 in a naked tree, which reacts on light differently from what is 

 tlie case with a tree in full leaf. Some of these influences are 

 well known to us ; such as the property of heat to call forth the 

 growth of plants, and of light to produce their green colour, and, 

 by decomposition, to cause warmth. But very many others, it 

 must be confessed, are entirely hidden from us. Thus we believe 

 that light and heat depend, like sound, on the oscillations of 

 an elastic medium ; but we know by experiment that there 

 exists a great difference among the rays of light, inasmuch as the 

 perception of light and heat for which our bodies are organised is 

 not the effect of one and the same ray of light, but that the 

 illuminating, heating, and the chemically acting rays may be 

 distinguished from each other, and that their refrangibility and 

 intensity are different. We know that plants receive heat, and 

 that it pervades them and mightily influences their processes of 

 life ; but the manner in which it receives modifications from the 

 internal and external form of plants, from the content of their tissue, 

 and even their social life, so to say, is still in many respects pro- 

 blematic, and as far as regards most garden establishments it 

 reduces itself practically to the before-mentioned rule : the more 

 light the better. Yet, nothing is more certain than this, that 

 they produce respectively vast variety of separate effects. Since 

 the time of Senebier it has been admitted that, next to its 

 influence on the process of suction by the roots, light augments 

 and accelerates the perspiration of plants, and causes their green 

 colour, besides affecting divers physical, physiological, and chemical 

 functions. But we are unable to apportion with certainty the 

 different operations to the different rays. M. Dumas 's observa- 

 tion, that green leaves produce a weaker photographic reaction 

 than other light-reflecting bodies, makes it probable that the 

 chemical rays operate principally the wonderful formation of 

 chlorophyll, and we discern the green of the leaves because they 

 reflect the yellow and blue (i. e. green), while the other elements 

 of the solar rays become absorbed. We must not suppose, how- 

 ever, that the phenomena are to be accorded exclusively to this or 



