568 LECTURE LVIII. 



and undergoing a chemical fermentation, in which oxygen is absorbed, 

 and a part of the mucilage contained in the seed is converted into sugar ; a 

 substance probably more nutritive to the young plant. The radicle shoots 

 downwards, and the seed leaves, or cotyledons, which are generally two, 

 although sometimes more or less numerous, raise themselves above the 

 ground, till in a short time they die and drop off, being succeeded by the 

 regular and more adult leaves. 



In every transverse section of a vegetable, we commonly discover at least 

 four different substances. The parts next to the axis of the tree or branch 

 consist of medulla or pith, which is supposed by some to be the residence 

 of the vegetable life of the plant ; but a tree may live for many years after 

 being 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 

 surrounded by the woody part, composed of fibres more or less strongly 

 compacted together, but not actually ramifying into each other in any 

 great degree, although there is reason to suspect some lateral communica- 

 tions between 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 vegetable ; 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 . The wood consists of a number of concentric layers or strata, 

 formed in successive 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 in as many different years ; the external parts 

 usually cracking, and allowing us at their divisions to observe their num- 

 ber, 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 office 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. Desfon- 

 taines* 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 vege- 

 tables : 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 are composed of single threads wound 

 * Mem. de 1'Instit. i. 478. 



