748 PHYSIOLOGY, 



may be freely exposed to those influences which are favourable 

 for the development of their foliage. 



3. The Medullary Bays. — The function which these rays per- 

 form is probably to assist the diffusion of a portion of the elabo- 

 rated sap from the bark and cambium layer through the wood, 

 in which certain of the secretions it contains are ultimately 

 deposited. 



4. The Bark. — The bark acts as a protection to the young and 

 tender parts within it. The inner part is generally believed to 

 convey the elaborated sap from the leaves downwards, in order 

 that new tissues may be developed, and the different secre- 

 tions deposited in the wood and in its own substance. The bark 

 frequently contains very active medicinal substances, and others 

 which are useful in the arts, &c. 



3. Of the Leaatis. — 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 absorption of fluid matters ; 

 3. The absorption and exhalation of gases ; and, 4. The forma- 

 tion of the various organic products and secretions of plants. 

 These functions they are enabled to perform through the influ- 

 ence of the 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. Exhalation of Watery Vapour by Leaves. — This process, 

 which is commonly termed transjnration, is considered to be 

 somewhat analogous to the perspiration of animals, but is little 

 more than evaporation. Its immediate object and effect 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 p. 741), 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 it is liable, have also an important influ- 

 ence upon the transj^iration of fluid matters. (See p. 741.) 



From some interesting experiments of M. Garreau on transpi- 

 ration by leaves, he was led to draw the following conclusions : — 

 1. The quantity of water exhaled by the upper and under sur- 

 faces of the leaves is usually as 1 to 2, 1 to 3, or even 1 to 5, or 

 more. The quantity has no relation to the position of the 

 surfaces, for the leaves, when reversed, gave the same results as 

 when in their natural position. 2. There is a correspondence 

 between the quantity of water exhaled and the number of the 

 stomata. 3. The transpiration of fluid takes place in greater 

 quantity on the parts of the epidermis where there is least waxy 

 or fatty matter, as along the line of the ribs. 



This transpiration of fluids is influenced to a great extent by 

 the varying conditions of the atmosphere as to moisture and 



