728 iECTUKE LVIII. 



spiral tube, like the spring of a bell, and capable of being easily uncoiled; 

 these, though they have been called air vessels, and supposed by some to 

 serve the purposes of respiration, are described by others as containing, during 

 the life of the plant, an aqueous fluid: and they are probably little more than 

 sap vessels, with an additional spiral coat : they are not found in the bark, 

 •nor in all species of plants; and it has thence been inferred that they are 

 not immediately necessary to the growth of the plant. The third kind are 

 the proper vessels of the plant, which are generally disposed in concentric 

 circles, and appear to be unconnected with the sap vessels, and to contain 

 the milky, resinous, and other peculiar juices, which are found in difterent 

 kinds of plants; for the sap is nearly the same in all, at least it is independ- 

 ent of the, gums and resin, which often distinguish particular plants; it con- 

 tains a certain portion of mucilage, and probably in some plants, as the 

 sugar maple, a considerable quantity of sugar. Mr. Mirbel has also made a 

 number of still more accurate distinctions respecting the structure of the dif- 

 ferent kinds of vessels. The circulation of the sap is not completely under- 

 stood; when an orifice is made near the root of a tree, it flows most copiously 

 from above : when near the summit, from below. Dr. Hope actually reverted 

 the natural course of the juices of a tree, without changing its position; by 

 inoculating a willow with two others, he completely united its existence with 

 theirs, and then, removing its roots, he found that its vegetation was sup- 

 ported by the juices of the two others. A tree may also be actually inverted, 

 and the upper part will strike root, the lower putting out branches and leaves. 



Plants perspire very considerably, and also emit a quantity of gases of 

 different kinds; they generate a slight degree of heat, which may be observed 

 by means of the thermometer, and by the melting of snow in contact with 

 them. The growth of every tree takes place at the internal surface of the 

 bark, not only the bark itself being formed there, but the wood also being 

 deposited by the bark; for Pr. Hope Separated the whole of the bark of a 

 branch of willow from the wood, leaving it connected only at the ends, so as 

 to constitute a hollow cylinder, parallel to the wood ; and he found that new 

 layers were formed within the bark; and in another experiment a part of the 

 wood, deprived of the bark, although protected from the air, was only 

 covered with new bark as it grew over from, the old bark above and below. 

 The layers of wood, which are added in successive seasons, and keep a 



