DESCRIPTION OF THE ACTION OF LIGHT ON LEAVES. 97 



nods in eternity ; but, whether we consider the scale of space, or of time, on which the 

 universe is constructed, we can see that our minds are so constituted as to be equally 

 unable to appreciate either extremity ; that we can attach no just idea to what is either 

 infinitely great or infinitely small ; and that therefore our views do not always justly 

 apply in natural events. Of one thing we may rest assured, that no matter how great 

 the periods that may be required for the phenomena of the universe to transpire, there 

 has been, and there will be time enough for their endless repetition. 



385. From considering what takes place when a green leaf is enlightened by the 

 sun, we are thus allured to pass on step by step to reflections on the history of the 

 solar system, and to changes which have happened to the earth. In the same way 

 that the stem of an exogenous tree is a lasting record and memorial of the returning 

 summers it has witnessed, each ring that we see pointing out the growth of one season, 

 and being, so to speak, an index of the amount of light which has been at play ; or, 

 when casting our eyes over the climates of the earth, we observe in the tropics a rank 

 vegetation, and trees and flowers flourishing all the year round ; or, coming to the tem- 

 perate zones, we find a hardier growth, and the soil only yielding its fruits to human 

 industry and skill ; or, passing towards the poles, the stunted plants, and lichens, and 

 mosses, and great plains covered with perpetual snow, and even in these inclement re- 

 gions all vital operations under the control of astronomical causes. Do not all these 

 serve to set forth the entire control which the solar rays have over these phenomena, 

 and teach us that the same kind of reasoning which applies to things taking place in 

 our time, applies also to things which have preceded it ; that if the section of an ex- 

 ogenous stem, or the climate-distribution of plants, point out a present connexion and 

 present relations with the sun, so do those fossils which are dug out of the ground 

 point to similar relations in former times; their magnitude and luxuriance indicate a 

 more brilliant ray. From the beginning of things no natural law has ever changed ; re- 

 sults are obtained in these times by the same operations or mechanism by which they 

 were obtained of old. As with us, so then, when the sunbeam falls on a leaf the yellow 

 ray is absorbed, and if carbonic acid is present, it undergoes decomposition, green mat- 

 ter is rapidly formed, a mixture of oxygen and nitrogen gases is emitted, and carbon, 

 oxygen, hydrogen, and nitrogen are fixed in the plant. The seat of this action is now, 

 as it was then, that face of the leaf which is exposed to the sky; and the nutritious 

 juice thus formed turning over to the under face of the leaf, is there concentrated by 

 the evaporatory action of the stomata. From these chemical changes mechanical 

 forces arise, and the nutritious sap is impelled downward, or brought into relation with 

 all parts of the plant. From it sugar, and gum, and starch, and woody fibre, albumen, 

 fibrine, &c., are formed, and these are lodged in various parts, or stored up for the 

 farther purposes of the economy. 



386. I regard, therefore, the sunlight, when acting upon plants, as operating exactly 

 in the same way as the chemical rays or radiant heat, when they produce their specific 

 phenomena. That, first of all, a certain absorption takes place, which seems to be un- 

 attended with any direct effect, and of the nature of which we have only an indistinct 

 idea ; that when this is over, the rays continuing to act, and the tissue of the leaves 



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