186 ABSORPTION BY FOLIAGE. 
The sap of the maple, and of other trees with deciduous leaves 
which grow in the same climate, flows most freely in the early 
spring, and especially in clear weather, when the nights are 
frosty and the days warm; for itis then that the melting 
snows supply the earth with moisture in the justest proportion, 
and that the absorbent power of the roots is stimulated to its 
highest activity. 
When the buds are ready to burst, and the green leaves begin 
to show themselves beneath their scaly covering, the ground 
has become drier, the absorption by the roots is diminished, and 
the sap, being immediately employed in the formation of the 
foliage, can be extracted from the stem in only small quan- 
tities. 
Absorption and Exhalation by Foliage. 
The leaves now commence the process of absorption, and im- 
bibe both uncombined gases and an unascertained but 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 
volumes 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 intonew 
combinations, and becomes a part of the solid framework of the 
vegetable,or a component of its deciduous products, it becomes 
evident 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 
certainly escapes through the buds, the nascent foliage, and the 
pores of the bark, and vegetable physiology tells us that there 
is a current of sap towards the roots as well as from them.* I 
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 cir- 
