14 STRUCTURE OF VEGETABLES. 



layer of the same kind. Hence the trunks of trees are 

 formed by layers of wood, which have been yearly deposited 

 around the centre, and have successively afforded a passage 

 for the sap, by means of the central or spiral vessels, to as- 

 cend into the branches and leaves. From the leaves the sap 

 descends through the vessels of the internal layer of the bark, 

 as in the former case, and in its descent gradually contributes 

 to the formation of the alburnum- for the next year. That 

 part of the bark, also, which has thus once served the pur- 

 poses of circulation, like the alburnum, is afterwards thrown 

 aside, and its place is supplied by a new layer formed on its 

 inside between it and the alburnum. Hence the large quan- 

 tity of thick and dead bark which is often accumulated upon 

 the outside of the trunk and branches of old trees. 



This is a slight sketch of some of the most important points 

 in the vegetable circulation. It appears from this, that the 

 principal seat of the growth and nutrition of plants is in the 

 bark and alburnum, and that all the new matter yearly added, 

 is deposited on the outside of the latter and the inside of the 

 former that the growth of one year is only subservient to 

 the circulation of the next, and is ever afterwards of use 

 merely in giving strength and stability to the trunk, in order to 

 support the increasing size and weight of the branches and 

 leaves. The wisdom and beauty of this provision, by which 

 that portion of the plant, which has become useless for every 

 other purpose, is thus made to answer a very important end, 

 are sufficiently obvious ; and it is rendered necessary by the 

 circumstance that plants do not, like animals, arrive at a defi- 

 nite size, and there cease, but go on growing to an indefinite 

 extent, and consequently require corresponding increase of 

 strength in those parts which are to support them. 



But besides this circulation, which is the most important 

 and interesting part of the vegetable economy, and is also the 

 best understood, the fluids of plants are no doubt subjected 

 in different parts to a variety of operations, and undergo many 

 changes. There are many other vessels besides those already 

 mentioned, whose office is not perfectly known, but which 

 contribute, probably, in some way, to the elaboration of the 

 different principles found in plants, such as gum, resin, 

 sugar, &/c., or are subservient to the performance of their 

 functions. The difficulty of accurately examining the minute 

 structure and organization of vegetables is very great, and it 

 is probable that we shall always remain ignorant of many 

 highly important parts of their physiology. 



