CHAPTER VIII. 



OF THE FUNCTIONS OF LEAVES. 



251. IT is in the Leaves, as already stated more than once, 

 that those changes are effected, which convert the crude fluid 

 absorbed by the roots (consisting as it does of little else than 

 water, in which is dissolved a very minute proportion of the 

 various matters existing in the surrounding soil,) into the proper 

 mice or nutritious sap, capable, not only of supplying to the 

 different parts of the structure the materials necessary for the 

 maintenance of their healthfulness, for the repair of injuries, and 

 for the production of entirely new parts, but also of furnishing 

 the ingredients of those several products, which the various tribes 

 of plants may be said almost to create from the elements around 

 them, and which are so valuable to Man as articles of diet, as 

 medicines, or as articles of use in his various manufactures. 

 Many of these will have to be considered hereafter, under the 

 head of Secretions; but it is interesting to observe here, that, 

 although almost every tribe of plants forms some substance 

 peculiar to itself, some of which are of a highly poisonous cha- 

 racter, whilst others are of the mildest and most wholesome 

 nature, they all originate in ascending sap, which is of a cha- 

 racter nearly uniform in each tribe. 



252. In this process of Elaboration, as this conversion has 

 been termed, several distinct changes are involved. The first is 

 the concentration of the fluid, by the loss of a considerable pro- 

 portion of its water ; so that the amount of solid matter, con- 

 tained in any quantity of it, is much greater than, before. This 

 is effected by a process, which resembles the perspiration of 

 Animals; a large quantity of watery vapour being given off, 



