102 OXYDATING PROCESSES REPLACED BY RADIANT HEAT. 



which all the nerves of sensation go, and from which all volitions and motions arise. 

 In plants, the attempt is to diffuse everything, to have no centre of action, but to exe- 

 cute upon the periphery. It is true that the lower order of animals are constructed 

 on this lype of diffusion, and it is interesting to see, when in more advanced tribes the 

 attempt at centralization begins, how rapidly it goes on to perfect development. The 

 appearance of an isolated organ of digestion a stomach is the signal of an isolated 

 organ of circulation a heart no matter though that heart may be a mere tube, as the 

 dorsal vessel of many insects. 



394. If thus, in animal existence, we find the various nervous machines divided off, 

 and the impressions of light, of sound, of taste, committed to separate apparatus, how 

 is it with plants ? The rays of the sun are the true nervous principle of plants ! And 

 herein we see how closely the type of surface action is observed. On all sides the 

 leaves present their thin lamina to the light, and offer a broad surface to the sky. On 

 these the rays fall, and direct the digestive and other processes. When they are car- 

 ried on in interior cavities, and the living system assumes a certain mass, an oxydating 

 machine is demanded in order to keep up the temperature of the whole by the burn- 

 ing of carbon, and lungs, or gills, or other suitable contrivances, are resorted to. But 

 the principle on which the vegetable organization is constructed is simple, and except 

 on those particular occasions to which reference has been made (387), an apparatus 

 of combustion is not wanted. For with the rays of light which come from the sun 

 there are also rays of heat, and these impinging on the outside surface, which is the 

 seat of all the vegetable activity, are absorbed, and bring up the temperature to the re- 

 quired point. What need is there for the burning of carbon or of hydrogen, when on 

 all sides radiant heat is pouring in ? Nature, with a provident care, has in all ordinary 

 leaves provided abundantly for this elevation of temperature, and when the direct beams 

 do not reach them, warm currents of air that rise from the heated ground are every- 

 where present. 



395. In a chemical point of view, it is the quality of light to produce the organiza- 

 tion of molecules, that is, to determine the positions of atoms as respects each other; 

 it is the quality of heat to determine their distances ; for the accumulation of caloric 

 in a body produces an expansion, a diminution of it a contraction. Light and heat, 

 therefore, have totally different offices to discharge, and the seats of their respective 

 actions are in most instances distinct. In horizontal leaves it is chiefly on the upper 

 face that light acts ; this, looking towards the sky, is ready to receive rays coming from 

 every part; it is on the under face that heat operates, directing in a great measure 

 through the stomata the evaporation of water. An advantage for these latter organs 

 is, therefore, gained. Exposed to the direct solar radiations, they would be under con- 

 ditions of perpetual disturbance, every cloud which passed over the sun would keep 

 them in a constant agitation, and interfere with their regularity of action. One of the 

 most prominent differences between light and heat is in their mode of propagation through 

 bodies; the force of the former is expended on the point upon which it falls; there is 

 nothing after the manner of conduction or lateral propagation; it is also the same with 

 the tithonic rays (Ap., 608). For this reason, when an image falls on a sensitive plate 



