Cree's Si^stem of Pnming Forest Trees. 447 



Early in spring, as the temperature of the atmosphere is 

 elevated, the sap ascends in the tree. It is absorbed from the 

 soil by the minute spongelets at the extremities of the capillary 

 rootlets of the root ; and it ascends through the root upwards. 

 In very young trees its ascent is through the pith, and also in 

 young branches ; but in trees even of a few years old, as well 

 as in old trees, it ascends neither through the bark, nor between 

 the wood and the bark, nor the pith, but through the concentric 

 layers of wood, and, in the greatest quantity, through those last 

 formed. 



The bud, which is formed in the preceding summer or au- 

 tumn, is supposed to be nourished previously to the evolution of 

 the leaf, by nutrient matter in the alburnum deposited in the 

 preceding autumn. After vernation takes place, and the leaves 

 are all expanded, the sap still continues to rise through the wood, 

 ascends to the branches, and from them to the leaves. The leaf 

 itself is formed of a vascular system of cellular tissue, covered 

 with an epidermis. The sap, when it ascends to the leaf, per- 

 spires, or throws off a large quantity of aqueous vapour. It is 

 then acted upon by light and air, in a manner unnecessary to be 

 here explained ; and it is at this part of the plant, and stage 

 of the course of the sap, that plants, in the opinion of some 

 philosophers, obtain the peculiar properties, aromatic, narcotic, 

 and the like. After the common sap has been thus changed 

 by the agency of the leaf, it is now called "proper juice"; it 

 accordingly descends in v/hat are called proper vessels, in con- 

 tradistinction to those in which it rises, and which in trees are 

 commonly situated in the bark. Trees, however, possess the 

 properties of adding their new wood either from the liber or the 

 alburnum, but it is generally deposited between the liber and the 

 alburnum of the last year, which is now being formed into wood. 

 The new vegetable matter thus formed, which was by Grew 

 termed cambium, differs in colour and properties from the proper 

 juice, and is regarded ar a secretion, separated from the proper 

 juice by the vascular structure of the liber or alburnum. And 

 it is in this state that it is fit for the formation of vegetable 

 matter, and each year forms the concentric cylinder of new 

 alburnum. 



It thus appears that leaves form the primary objects of vege- 

 table functions, and that they form the organs of communica- 

 tion between the wood and the bark, and are the chief cause 

 of the ascent of the sap after they have expanded. The sap, 

 indeed, does ascend, and even in greater quantity, before the 

 leaves have expanded ; but this is an effort which the tree is 

 known to possess only for a limited period. And the experi- 

 ments of Dr. Hales and others made upon plants by divesting 

 them of their leaves, clearly show the important functions which 



18-tl.— IX. 3d Ser. c g 



