2o0 VEGETABLE ORGANOGRAPHY. 



CHAPTER III. 



OF THE LEAVES OF VASCULAR PLANTS. 



Section I. 

 Of the General Structure of Leaves. 



The Leaves are, as every one knows, those expan- 

 sions, generally flat, of forms so various, which spring 

 laterally from the stem or branches of plants, and form 

 oneof their principal ornaments. Turpin calls them in 

 a more general manner by the name of the appendicular 

 organs of plants, joining under this term not only the 

 leaves in their natural state, but all the other lateral or- 

 gans of stems, which are only modifications of them, as 

 we shall shortly see. We shall here consider them in 

 their ordinary state. 



When they are studied with regard to Physiology, 

 it is found that they are the principal organs of the 

 aqueous evaporation, of the decomposition of the gases 

 and juices, and consequently the most essential agents 

 of nutrition. If we consider them anatomically, as it is 

 here our intention to do, we perceive that a leaf is the 

 expansion or spreading out of one or several fibres, 

 which, detaching themselves or springing from the mass 

 of the stem, spread out in such a manner that each 

 vessel is separated from all the others, and has its orifice 

 more or less isolated. If this fundamental idea is just, 

 its development ought to explain to us the entire struc- 



