186 pHytotomy. ^ 



instance, white in the Birch and in the Melaleuca teucaden- 

 dron ; of a golden yellow in the Aucuha Japonica ; in a more 

 advanced age, it exhibits slight rents ; by and by it thickens 

 into a substance resembling cork, and, in consequence of the 

 increasing thickness of the stem, it is thrown off, as is very 

 distinctly seen in the Platanus. This latter phenomenon is a 

 proof that the epidermis, in this condition, is no longer orga- 

 nised, nor is of any use to the tree. However, in a consi- 

 derable number of trees, it has a certain permeability, by 

 means of which the ingredients of the air can have an in- 

 ^uence through it upon the interior layers. 



293. 



The proper rind, which, in its earliest state, is of a green 

 colour, assumes other hues at a later period. Its cells contain 

 concentrated and peculiar juices, which, by being deposited 

 upon its sides, make them impenetrable to the eye. Many 

 of these cells are so extended by the sap, that they appear 

 like proper sap-vessels; because they are stretched out in 

 length, are surrounded by a very fine cellular texture, and 

 are closed at both ends. In Pines, in Celadine, and also in 

 the Rue species, these peculiar sap-passages can be most dis- 

 tinctly observed. 



The rind cells proceed horizontally through the interior lay- 

 ers of the stem towards the inmost, and open by this means 

 a very remarkable connection between all these layers, which 

 is of consequence to the explanation of many phenomena. In 

 most trees, however, this connection is interrupted at certain 

 periods of their growth. The juices which rise in the inner 

 bark, pass more readily the higher they ascend, intq the gene- 

 rative sap, a kind of slimy organizing fluid. This sap, forcing 

 its way from the outermost layers of the inner bark, tears 

 the rind cells asunder, and fills the space which is thus form- 

 ed between the rind and the inner bark. It is in this way 

 that at the periods I have mentioned the rind is loosened, 

 and a foreign branch, or a bud of another tree, can now much 

 more easily be brought into this space, that it may derive its 

 nourishment from the generative sap, and as it were take ixx)t 



