32 THE LIGHT OF THE SUN IS THE CAUSE OF THE FLOW OF THE SAP. 



99. Thus, in the vegetable world, if we commence our examination with the lowest 

 orders of cellular plants, and pass successively through the mosses and ferns to the 

 flowering plants, we see the same physical cause in constant operation. They all ob- 

 tain their liquid and their solid food through the agency of capillary attraction the 

 former from the soil, and the latter, in higher tribes, from the air. It is, moreover, in 

 virtue of its action as a capillary mass, that the ground becomes uniformly moist ; 

 and hence, through the operation of the same power, fluid matter is first brought to the 

 plant, and then distributed through it. It is in virtue of its capillary diffusion through 

 atmospheric air that carbonic acid gas is brought in contact with the leaves ; and when 

 there reduced, it is through the same force that the carbon is distributed to the grow- 

 ing parts. Of the energy of this force there is abundant evidence, not only that it can 

 cause the gradual percolation of juices through small structures, such as the minutest 

 moss, but also lift the sap to the tops of the highest forest trees, and, indeed, far higher 

 were it necessary, and drive it back again to the roots, irrespective of the agency of 

 gravity. Even gaseous substances, as is shown experimentally in the Appendix (Cn. 

 XL), pass into one another with a force greater than the pressure of a column,of wa- 

 ter seven hundred feet high ; so that, to elevate the sap in a tree, or to drive the blood 

 in an animal, is an insignificant demand on the energy which this force could put forth. 



100. In conclusion, we here again remark the influence which the imponderable 

 principles exert in directing all kinds of movements on the surface of our globe. The 

 sap rises in a tree because the sun shines ; it is the light of that central orb which pro- 

 duces even these movements in plants. Indirectly, it is true, chemical affinities or elec- 

 trical agencies are brought into operation, but the prime mover of the machine is the 

 light, which produces a mucilaginous body, which is different in composition in differ- 

 ent plants, and which constitutes their proper juices. 



101. The cause of the movement of the sap injlowering plants both of the rue of \, 

 the crude sap upward, and of the descent of the elaborated sap downward is the light 

 of the sun, which effects the decomposition of carbonic acid gas. 



