08 VEGETABLE ORGANOGRAPHY. 



envelope. It happens thus in all tr^es, that after the 

 time when the transverse distension has favoured the 

 formation of an epidermis, it is succeeded by another, 

 when, from the same cause, the epidermis is destroyed 

 by the cracks of the bark. 



It is the cuticle wliich always bears either the stomata, 

 which are only seen well by the microscope, or the hairs 

 which so often cover the surfaces of leaves: of both these 

 we shall speak very shortly. We are obliged to men- 

 tion them here for the purpose of remarking, that when 

 the cuticle is destroyed, these organs likewise are. The 

 epidermis, properly so called, which is formed by the 

 drying up of the cellular tissue, never bears either hairs 

 or stomata. This circumstance tends to confirm the 

 difference between these two membranes, hitherto con- 

 founded under a common denomination. 



The cuticle of young branches has a tendency, in 

 general, to tear, separate, or detach itself in a longitu- 

 dinal direction, which is that of its growth ; but when 

 the elongation has ceased, and as the increase in diameter 

 becomes perceptible, the cellules, which upon drying 

 form the epidermis, are drawn in a transverse direction, 

 so that instead of being elongated longitudinally as they 

 were at first, they become so transversely : it results, 

 therefore, that they are more easy to break across than 

 lengthways, for they present in this direction fewer 

 partitions than in the other ; thus the same reason which 

 makes all organs which increase in length, more easy to 

 be split in a longitudinal direction, causes the epidermis, 

 which is distended transversely, more easy to be split in 

 a transverse direction ; so that the epidermis of the 

 Birch, Cherry, and generally of all smooth trunks, divides 

 nearly circularly across. In trunks marked with chan- 

 nels and longitudinal canals, as the Vine, the epidermis 

 preserves, on account of its longitudinal inequalities, 



