THE LEAVES OF VASCULAR PLANTS. 301 



present at the period of their fall ; but the real cause of 

 this fall is always the existence of an articulation. 



Leaves are in general called Caducous, when they 

 fall off at the end of the first year, and Persistent 

 when they last longer : we designate in particular by the 

 name of Evergreen trees, those which have persistent 

 leaves. But it must be remarked, that these terms, 

 deduced from appearance, are far from being correct : 

 leaves destined to fall off ought to be called Falling 

 leaves, and among these we may distinguish : — 1st. Those 

 which fall off the first year, or Annual ones; — 2d. 

 Those which fall off the second year, after the develop- 

 ment of new leaves, or Biennial ones; — 3d. There are 

 some falling leaves, as those of Firs, which remain two, 

 three, or a greater number of years, but which ought 

 not to be confounded with persistent leaves, although 

 both constitute the permanent foliage of evergreen trees 

 and shrubs. 



Section XII. 



Of the Functions of Leaves, and of the means of sup- 

 plying their place in Leafless Plants. 



The functions of leaves are objects more physiological 

 than anatomical, and which we ought only to examine in 

 a summary manner. 



1st. We have seen that all the structure of this organ 

 has for its result the isolation of the extremities of the 

 sap-vessels, leaving, however, cellular tissue between 

 each of them. The open extremities of the vessels or 

 intercellular passages, or the stomata, serve in general 



