VEGETABLE PHYSIOLOGY. 31 



from the extremity of the branches, 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 central 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 continuation of the 

 cellular tissue, which forms its principal bulk, of a distribution 

 of the alburnum or sap wood, of a small portion of the lignum or 

 heart wood, and of the upper extremities only of the cortical 

 vessels : all of a peculiarly minute and fine texture, form- 

 ing a delicate net work ; the whole being covered by the 

 epidermis, the size of the leaf varying, from the smallest pro- 

 portion that can be distinguished by the naked eye, to a magni- 

 tude that almost exceeds belief, namely, to several feet in 

 diameter. 



The functions of this very interesting organ may be considered 

 the most important of any which are connected with the preser- 

 vation of the plant. To illustrate these, it is to be recollected 

 that the nutritive ingredients, when first absorbed, are only in 

 the simple state, held in solution by a considerable proportion of 

 a watery fluid ; and require the aid of a further process before 

 they possess the consistency and the chemical properties requisite 

 to produce the various secretions which are to contribute to the 

 nourishment and preservation of the plant. The agent, to effect 

 these important changes, are the leaves ; and the process, by 

 which those changes are accomplished, is, in the first place, by 

 evaporating a considerable proportion of the watery part of the 

 simple sap as it is conducted from the roots, by which its fluidity 

 is diminished ; and, in the next, by absorbing, or taking up from 

 the atmosphere, a certain proportion of its oxygen, caloric, light, 

 and of various nutritive materials, held in solution in that element ; 

 by which, through chemical agency, new principles are commu- 

 nicated to the fluid, now denominated, the proper juice or true 

 sap,' previously to its descent, to supply the different parts of the 

 plant with renovating secretions. 



To promote this object, the surface of the leaf is rendered 

 broad and extensive, the tubes and cells exquisitely fine and 



