ABSORPTION BY FOLIAGE. 183 



Absorption, a/nd ExTwlation hy Foliage. 



The leaves now commence the process of absorption, and im 

 bibe both uncombined gases and an unascertained, bnt probably 

 inconsiderable, quantity of aqueous vapor from the humid atmos- 

 phere of spring which bathes them. 



The organic action of the tree, as thus far described, tends to 

 the desiccation of air and earth ; but when we consider what vol- 

 umes of water are daily absorbed by a large tree, and how small 

 a proportion of the weight of this fluid consists of matter which, 

 at the period when the flow of sap is freest, enters into new com- 

 binations and becomes a part of the solid framework of the 

 vegetable or a component of its deciduous products, it is evi- 

 <ient that the superfluous moisture must somehow be carried back 

 again almost as rapidly as it flows into the tree. At the very 

 commencement of vegetation in spring, some of this fluid cer- 

 tainly escapes through the buds, the nascent foliage, and the pores 

 of the bark, and vegetable physiology tells us that there is a cur- 

 rent of sap towards the roots as well as from them.* I do not 

 know that the exudation of water into the earth, through the 

 bark or at the extremities of these latter organs, has been proved, 

 but the other known modes of carrying o£E the surplus do not 

 seem adequate to dispose of it at the almost leafless period when 

 it is most abundantly received, and it is possible that the roots 

 may, to some extent, drain as well as flood the watercourses of 

 their stem. Later in the season the roots absorb less, and the 

 now developed leaves exhale an increased quantity of moisture 

 into the air. In any event, all the water derived by the growing 

 tree from the atmosphere and the ground is parted with by tran- 

 spiration or exudation, after having surrendered to the plant the 

 small proportion of matter required for vegetable growth which 



* " The elaborated sap, passing out of the leaves, is received into the inner 

 bark, .... and a part of what descends finds its way even to the ends of the 

 roots, and is all along diffused laterally into the stem, where it meets and min- 

 gles with the ascending crude sap or raw material. So there is no separate 

 circulation of the two kinds of sap ; and no crude sap exists separately in any 

 part of the plant. Even in the root, where it enters, this mingles at once with 

 f3ome elaborated sap already there." — Gbat, How Plants Qrow, § 278. 



