THE LEAVES OF VASCULAR PLANTS. 303 



performed in the leaves, and tliat these are the organs in 

 particular which form the Cambium, or the juice which 

 nourishes and develops the wood and bark. This im- 

 portant use of leaves may be recognised in the experi- 

 ments when the leaves of a branch are in part or entirely 

 removed, when we see its growth diminish in proportion 

 throughout all the part below that where the leaves were 

 taken off. From the preceding circumstances, we may 

 conclude how idle are the discussions so freuqently 

 introduced into physiological and agricultural works, 

 to know if the leaves nourish the plant more or 

 less than the roots ; it is just as if it were to be 

 asked whether a man's lungs nourished him more or less 

 than his stomach. Nutrition is a phenomenon perfected 

 and performed by several organs. The roots contribute 

 their part, and the leaves theirs. 



7th. Leaves, in presenting to the winds two resisting 

 surfaces, more or less considerable, tend to produce an 

 almost continual agitation in the branches of trees ; and 

 we know, from some beautiful experiments of Mr. 

 Knight, that the motion of the branches facilitates the 

 progress of the sap and the growth of the trunk ; it is, 

 perhaps, one of the causes which makes trees with large 

 leaves to grow more rapidly. 



8th. A great number of leaves also serve for the 

 secretion of different particular juices, according to the 

 nature of the glands with which they are provided. 



9th. Several serve for the particular shelter and pro- 

 tection either of the flowers or fruit, or of the buds 

 situated in their axils. 



These different functions are so important that the 

 leaves constitute the truly active portion of vegeta- 

 tion ; and it is difficult to understand how it is possible 

 for plants to exist devoid of these essential organs. 

 This, however, happens under different forms, and we 



