192 RETURN OF THE ELABORATED SAP. 



of the earth's history, a much larger amount of carbonic acid 

 existed in the atmosphere. At the time of the existence of those 

 vast pine-forests, which, in their decomposed state, supply us 

 with such an enormous amount of that most valuable article of 

 England's wealth coal an article more really valuable to her 

 than the mines of Peru, scarcely any land animals seem to 

 have existed ; and these were of kinds, which are now found to 

 be capable of breathing a comparatively impure air. The great 

 luxuriance of these forests, as indicated by the vast amount of 

 their remains, and by more perfect specimens which have been 

 preserved to us, has led to the opinion that, at the period of their 

 growth, more carbonic acid existed in the atmosphere than at 

 present ; and that, in fact, it has from that time been gra- 

 dually undergoing purification, by the processes of vegetable 

 growth : and has at last become fit for the residence of the higher 

 animals. 



Return of the Elaborated Sap. 



299. The crude ?ap is brought into the leaves, by the vessels 

 which are connected with the woody portion of the stem ; and 

 these branch out and subdivide in the veins, so as speedily to 

 distribute the fluid over the whole surface. After undergoing 

 the various changes now described, it is collected again by a 

 system of vessels, which lie nearer the lower surface, and which 

 communicate with the bark. These are principally of the kind 

 formerly described as vessels of the latex; and through these the 

 descending sap, now completely changed in its properties, is 

 returned to the stem. This fluid contains the materials of all 

 the products of the vegetable system ; the elements of the or- 

 ganised tissues, the secretions which give solidity and toughness 

 to the wood, those which occasion the delicious odours that so 

 abound among plants, and those which supply so many useful 

 and important products, with which the comfort and luxury of 

 Man are largely connected. All these are entirely dependent 

 on the action of the leaves ; and the action of the leaves is 

 dependent upon the supply of that amount of light and heat, but 

 especially the former, which each species of plant requires. 



