796 FUNCTIONS OF LEAVES.—EXHALATION OR TRANSPIRATION, 
3. OF THE LEAVES OR PHyLtLomes.—The essential functions 
of the leaves are :—(1) The exhalation of the superfluous fluid 
of the crude sap in the form of watery vapour ; (2) the absorp- 
tion of fluid matter ; (3) the absorption and exhalation of gases ; 
and (4) the formation of the organic compounds which are con- 
cerned in the development of new tissues, and in the formation 
of the various secretions of plants. These functions they are 
enabled to perform through the influence of heat, air, and light, 
to which agents, by their position on the ascending axis of the 
plant, and by their own structure, they are necessarily, under 
ordinary circumstances, freely exposed. 
(1) Exvhalaiion of Watery Vapour by the Leaves.—The im- 
Fre. 1147. 
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Fig. 1147. Phaseolus multiflorus. Longitudinal section through the apical 
region of the stem of an embryo. ss. Apex. pb, pb. Parts of the two 
first leaves. k, & Commencement of their axillary buds. (After Sachs.) 
mediate object and effect of this process, which is commonly 
termed transpiration, is, the thickening of the crude sap, and 
the consequent increase of solid contents in any particular portion 
of it. This transpiration of watery vapour, as already noticed 
(see page 787), takes place almost entirely through the stomata, 
and hence as a general rule the quantity transpired will be in 
proportion to their number. The presence or absence of a true 
epidermis, and the various modifications to which this is liable, 
have also, as already noticed (page 786), an important influence 
upon the transpiration of fluid matters. 
From some interesting experiments of M. Garreau on tran- 
spiration of leaves, he was led to draw the following conclusions :—— 
1. The quantity of water exhaled by the upper and under surfaces 
