ON VEGETATION. 569 



into a 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 con- 

 taining, during the life of the plant, an aqueous fluid : and they are pro- 

 bably 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 gene- 

 rally 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 different kinds of plants ; for the sap is nearly the same 

 in all, at least it is independent of the gums and resin, which often distin- 

 guish particular plants ; it contains a certain portion of mucilage, and pro- 

 bably 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 different kinds of vessels. The circulation 

 of the sap is not completely understood ; 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 supported 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 con- 

 tact with them. The growth of every tree takes place at the internal sur- 

 face of the bark, not only the bark itself being formed there, but the wood 

 also being deposited by the bark ; for Dr. 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 expe- 

 riment 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 register of the age of the tree, are very easily observed 

 when it is cut across ; sometimes as many as 400 have been found in firs, 

 and oaks are said to have lived 1000 years. 



Mr. Knight f has inferred, from a great variety of experiments, that the 

 sap, either usually or universally, ascends through the wood into the 

 leaves, and then descends through the bark to nourish the plant. The 

 leaves seem to be somewhat analogous to lungs, or rather to the gills of 



* Bullet, de la Soc. Philom. No. 60. Journal de Phy. lii. 336. Anatomie et Phy- 

 siologic Vgget. 2 vols. Paris, 1815. 



f His papers are in the Ph. Tr. 1795, p. 290; 1799, p. 195 ; 1801, p. 333 ; 

 1803, p. 277; 1804, p. 183. 



