734 PHYSIOLOGY. 



connected with it and with the bark of Dicotyledons, is the 

 vitally active layer of cells called the cambium layer, from 

 which are annually formed new layers of wood and inner bark. 

 The cells of the cambium layer are filled in the spring and 

 at other seasons when growth takes place, with elaborated sap, 

 or that sap which contains all the materials necessary for the 

 development of new structures. Great diiFerences of opinion 

 exist amongst botanists as to the exact manner in which wood is 

 deposited, but they are nearly all agreed that the materials from 

 which it is formed are elaborated in the leaves, and that hence 

 without leaves there can be no additions to the wood, and in 

 proportion to their amount so will be the thickness of the wood. 

 It is necessary, therefore, that the process of pruning timber trees 

 should be carefully conducted. As the formation of the elabo- 

 rated sap also depends upon the proper exposure of the crude 

 sap to air and light, and as new wood is formed out of the 

 materials it contains, it is necessary when trees are planted for 

 timber that they should be placed at proper intervals, in order 

 that they may be freely exposed to those influences which are 

 favourable for their development. 



3. The Medullary Rays. — The principal function which these 

 rays perform is, to convey a portion of the elaborated 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 ; it also conveys the elaborated sap from 

 the leaves downwards, in order that new tissues may be developed, 

 and the different secretions deposited in the wood and in its 

 own substance. The bark frequently contains very active medi- 

 cinal substances, and others which are useful in the arts, &c. 



3. Of the Leaves. — 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 formation of the various organic products and secre- 

 tions of plants. These functions they are enabled to perform 

 through the influence of the air and light, to which agents, by 

 their position on the ascending axis of the plant, they are 

 necessarily, under ordinary circumstances, freely exposed. 



1. Exhalation of Watery Vapour by Leaves. — This process, 

 which is commonly termed transpiration, is considered to be some- 

 what analogous to the perspiration of animals. Its immediate 

 object is, the thickening of the crude sap, and the consequent in- 

 crease of solid contents in any particular portion of it. This trans- 

 piration of watery vapour, as already noticed (see p. 728), 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 



