THE LEAVES OF VASCULAR PLANTS. 235 



lular tissue enlarging interposes itself between the fibres, 

 separates them, and at the same time the hairs ; so that, 

 even when they are not thrown off, as sometimes hap- 

 pens, the old leaves appear less hairy than young ones, 

 and the leaves of plants growing in a fertile soil are less 

 so than those which have grown in a sterile place ; con- 

 sequently, in general, cultivated plants are less hairy 

 than wild ones. 



The mesophyllum comprehends probably two systems 

 of organs, but anatomy has not yet been able to dis- 

 tinguish them, viz, — 1st, a system which receives the 

 ascending sap, brings it into contact with the air for its 

 elaboration, and permits the exhalation of the super- 

 abundant parts ; — 2d, a system which receives the 

 elaborated sap and conducts it back to the stem, where 

 it serves for nutrition. Physiological phenomena prove 

 the existence of these two functions in leaves, but anato- 

 mical observation has not distinguished them ; it is not 

 even known if the two fimctions are performed alter- 

 nately by the same organs, or if, wliich is more likely, 

 they are the consequence of two different systems. 



The two surfaces of leaves are composed of true 

 cuticles, and all that we have said of that organ is appli- 

 cable to them ; in particular, it may be observed, on 

 comparing species together, that the cuticles are the 

 more easily raised as the fibrous tissue is less abundant 

 in proportion than the cellular, and consequently as the 

 number of stomata of one surface is less. In other 

 terms, the cuticle is easily raised in plants where the 

 cellular tissue is abundant ; thus, very herbaceous leaves 

 are more easy to peel than fibrous ones ; but when the 

 different surfaces of the same species are compared, this 

 law is modified by a contrary one — viz. the cuticles of 

 leaves are more easily raised where the most stomata 

 are found, because the evaporation being greater there. 



