158 VEGETABLE ORGANOGRAPHY. 



Striped transversely, in soft wood, and dotted in hard. 

 The only organic difference which exists between the 

 wood and the alburnum is, that the interior of the cel- 

 lules, and perhaps of the vessels, is usually encrusted in 

 the former, whilst it is empty, or filled with less solid 

 juices in the latter, which, consequently, has the mem- 

 branous tissue more transparent. Dutrochet has proved 

 that the hardness of different woods, and of the wood 

 and the alburnum, results from the nature of the juice 

 deposited in their tissue, and not from the tissue itself, 

 which appears identical. By heating Ebony in nitric 

 acid, the black matter is dissolved out, and the tissue 

 remains of a pearly white colour. It is the same with 

 all coloured woods : the tissue of the Box and Poplar, 

 although these woods differ much in their density, be- 

 comes similar when the matter with which it is filled is 

 dissolved out by nitric acid. 



Each woody layer, in Exogenous trees, is the product 

 of the vegetation of one year ; but Duhamel thinks that 

 it is not formed all at once : he says that each layer is 

 itself composed of an indefinite number of little parallel 

 layers, or rather, that it is continually growing during 

 the whole year, but with more or less activity at dif- 

 ferent seasons. Duhamel has demonstrated his opinion 

 by an apparently simple experiment ; — in the beginning 

 of the spring he raised up the bark of a young tree, 

 placed a thin piece of tin between it and the wood, and 

 laid it (the bark) back again upon the wound; he repeated 

 this every fortnight, as long as the bark could be sepa- 

 rated from the wood ; at the end of the Autumn, having 

 cut down the tree, he found that each plate of tin was 

 covered by a quantity of wood, which was larger accord- 

 ing to the length of time it had been placed there. 

 This experiment, although in appearance conclusive, 

 may conceal several errors ; and it is likely that it was 



