ON STEMS. 49 



There are some trees whose bark is of so eiastic and 

 yielding a nature, that it does not harden for a considera- 

 ble number of years. The bark of the cork-tree, for in- 

 stance (which is the part commonly called cork,) does not 

 begin to harden till after the age of seven years : care is 

 taken to strip it off for the use of the arts before that pe- 

 riod. The bark of the plane-tree is, on the contrary, of 

 so hard and inflexible a texture, that it cannot expand, 

 but splits and falls off every season. These two species 

 of trees, the cork and the plane-tree, form the two ex- 

 tremities in the scale of varieties of texture in the nature 

 of bark. The cuticle or external coating of bark is not 

 confined to the stems and branches, but spreads itself 

 over the leaves, and every part of the surface of the plant 

 which is of a green color. 



Emily. But the bark of trees is not of a green color, 

 Mrs. B. ? 



Mrs. B. Recollect that the cuticle is an envelope, 

 which lasts seldom more than a twelvemonth. In those 

 parts of a plant which are of longer duration, such as the 

 stem and branches of trees, the cuticle decays and peels 

 off; and its place is supplied by the epidermis, a coating 

 formed by the desiccation of the external part of the cel- 

 lular tissue which has been left exposed to the air. The 

 epidermis, therefore, is not green. 



Aquatic plants form the only exception, these having, 

 properly speaking, no epidermis. 



If you pass a silver wire or blade completely through 

 the bark of a tree, the new internal layers, as they are an- 

 nually formed, will gradually push it outward, till at length 

 the internal coat becoming external, the wire will fall off. 



Caroline. That is, no doubt, the cause why inscrip- 

 tions on the bark of trees are, in the course of time, effac- 

 ed : the new bark does not grow over them it is true, but 

 growing under them, the inscription becomes distended, 

 and when the bark gives way, it will most readily split 

 and fall off where the inscription has already wounded it. 



239. What is said of the bark of the cork-tree'? 240. What is 

 said of the bark of the plane-tree 1 ? 241. What is said of the cork 

 and plane trees comparatively 1 ? 242. Over what parts of the plant 

 does the cuticle of bark extend! 243. When the cuticle decays, by 

 what is its place supplied! 244. How is the epidermis formed! 

 245. What is said of aquatic plants! 246. What experiment illus- 

 trative of the manner in which bark grows can be made with a silver wire! 

 o 



