66 VEGETABLE ORGANOGRAPHY. 



There is great diversity among plants of different 

 species, and among the organs of the same plant, with 

 regard to the facility with which we can raise the conti- 

 nuous pellicle form.ed by the external wall of the cellules : 

 in general, that of the lower surface of leaves can be 

 detached more easily than that of the upper ; that of 

 leaves of an herbaceous or fleshy tissue, more easily 

 than that of dry and woody ones ; that of foliaceous 

 parts, more than that of the sexual or petaloid ; that 

 of surfaces furnished with stomata, than that where they 

 are absent ; that of organs exposed to the air, than that 

 of those which are submerged or subterranean ; that of 

 smooth or slightly downy surfaces, than that of those 

 abundantly covered with hair, &c. The various com- 

 binations of all these elements, determine the numerous 

 and easily comprehended differences between all plants. 



Section III. 

 Of the Epidermis of Old Trunks. 



As soon as a young shoot has attained its natural 

 size, it ceases to increase in length, and commences to 

 expand, following the laws which we will hereafter 

 examine. The primitive cuticle, which has fulfilled for 

 a certain time the function for which it was destined, 

 and which is not destroyed by the fall of the organ, as 

 happens in the case of leaves, flowers and fruits, — the 

 cuticle, I say, of the branches or living stems is found in 

 a particular state : it becomes at first a little opaque ; 

 then dries up, exfoliates, or cracks, either from the con- 

 tinuance of the evaporation and the action of the air, or 



