64 ON VEGETABLE PHYSIOLOGY. 



though they allow its use in making good the 

 nutritive exhaustion which the rapid growth of 

 young shoots never fails to produce. 



From the stem or trunk, progressively emanate 

 the several branches of the tree or plant ; the 

 structure of which corresponding exactly with 

 that of the stem, we shall proceed to the leaves 

 which hold so important a place in the economy 

 of vegetation. 



This part of the plant, which contributes so 

 much to its beauty (though infinitely diversified 

 in its forms), is in all cases similar in its organi- 

 zation as well as in its functions. It consists of a 

 thin and flat substance usually of a green colour, 

 issuing generally from the extremity of the bran- 

 ches, though sometimes from the stem ; and is to 

 be distinguished by the sight or touch, into an 

 upper and under surface, a base, a midriff or 

 centre line of division, and into lateral lines, or, 

 as they have been improperly termed, lateral 

 nerves. 



In its structure, the leaf is made up of a con- 

 tinuation of the cellular tissue, which forms its 

 principal bulk, of a distribution of the alburnum 

 or sap wood, of a small portion of the heart wood 

 and of the upper extremities only of the cortical 

 vessels; all of a peculiarly minute and fine 

 texture, forming a delicate net work ; the 

 whole being very thinly covered over by the 

 epidermis, the size of the leaf varying, from the 



