ON VEGETABLE PHYSIOLOGY. 65 



smallest proportion that can be distinguished by 

 the naked eye, to a magnitude that almost ex- 

 ceeds 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 preservation of the 

 plant. To illustrate these, it is to be recollected 

 that the nutritive ingredients, when first absorbed, 

 are only in their 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 proper- 

 ties 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 accomplish- 

 ed, is, in the first place, by evaporating a con- 

 siderable 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 taken 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 communicated to the 

 fluid, now denominated, the proper juice or true 

 sap, previously to its descent, to supply the differ- 

 ent parts of the plant with renovating secretions, 



To promote this object, the surface of the leaf 



