OK VEGETATION. 727 



ing in great measure deprived of its medulla. The pith is of a loose and 

 light spongy texture ; it sends a ramification into each branch and each leaf, 

 where it appears to serve also as a reservoir of moisture. The pith is sur- 

 rounded by the woody part, composed of fibres more or less strongly com- 

 pacted together, but not actually ramifying into each other in any great 

 degree, although there is reason to suspect some lateral communications bcf 

 tween them. They are interrupted, at certain intervals, in many trees, by- 

 fibres, in a radiating direction, forming what is called the silver grain. Like 

 the bones in animals, the wood constitutes the strongest part of the vege-- 

 table; and like them too it is in a certain degree furnished with vessels. It 

 has even been supposed by some, that the fibres themselves are distinct 

 tubes, and by others, that the interstices between them serve the purpose 

 of vessels, but neither of these opinions is at present generally received. Th« 

 wood consists of a number of concentric layers or strata, formed in succes- 

 sive years; the external part, which is last formed, is called the alburnum, or 

 white wood, and this part is the most vascular. The bark encompasses the 

 wood; and this also consists, in trees, of several layers, which are produced ia- 

 as many difterent years ; the external parts usually cracking, and allowing us 

 at their divisions to observe their number, the inner layer only being of 

 immediate use. This layer is called the liber, and since this material was 

 once used instead of paper, the Romans called a book also liber. The bark 

 consists of fibres of the same kind as the wood, but more loosely connected. 

 It is covered by the cuticle, which extends itself in a very great degree, as 

 the growth of the vegetable advances, but at last cracks, and has its ofiice 

 supplied by the outer layers of bark. Between the bark and the cuticle a 

 green pulpy substance, or parenchyma, is found, which seems to be analogous 

 to the rete mucosum, interposed between the true skin and the cuticle in 

 animals. Mr. Desfontaines has observed, that in palms, and in several other 

 natural orders of plants, the annual deposition of new matter is not confined to 

 the external surface, but that it takes place in various parts of the plant, 

 as if it were composed of a number of ordinary stems united together. - 



There are three principal kinds of vessels in the different parts of vegeta- 

 bles: the sap vessels, which are found both in the wood and in the bark, 

 although their nature appears to require further examination: secondly, the 

 air vessels, or tracheae, which arc composed of single threads wound into a 



