THE CUTICLE AND EPIDERMIS. 69 



the property of splitting lengthways. In tubercles, 

 exostoses, and generally in all round parts, and where the 

 growth is in all directions, the facility of rupturing or 

 splitting the epidermis is likewise equal in all directions. 



I have given the name of Lenticels {Lenticelles) to 

 certain little oval spots which are seen on the bark of 

 several trees, especially of the Birch : their use and 

 history will be the object of one of the following chap- 

 ters ; but I mention them here because they may serve to 

 verify all that I have said. In their young state they are 

 oval longitudinally ; by degrees they become round, on 

 account of the thickening of the branch ; and they finish 

 by elongating transversely. The cuticle or epidermis of 

 the branch, of which these organs form part, must undergo 

 the same modifications as regards the form of its cellules. 



The use of the epidermis on trunks must be, in ge- 

 neral, to protect the cellular envelope: this is performed, 

 according to circumstances, in three ways : — 



1st. The epidermis stops or diminishes the evapor- 

 ation ; and the absence of all evaporating pores in this 

 membrane sufficiently explains the cause. 



2d. The epidermis opposes the decomposition which 

 would be caused by the external humidity : the earthy, 

 and frequently even silicious nature of this membrane, 

 perfectly gives the reason of this result. 



3d. The epidermis may also, in some cases, prevent 

 the frost from reaching the bark. This last eflFect is 

 especially observed in trees, the epidermes of which are 

 numerous : each of them retains a layer of enclosed air ; 

 and they form so many envelopes, which prevent the 

 bark from acquiring an equilibrium of temperature 

 with the circumambient air. 



Thus the Birch, which has more epidermes than any 

 other European tree, ascends highest upon the Alps, 

 and reaches nearest the frozen regions of the pole. 



