88 DECOMPOSITION TAKES PLACE IN THE STEM. " 



power, is a considerable step towards a clear conception of tlie nature of 

 that power itself. If the phenomena of endosmose can hereafter be 

 clearly and indubitably traced to the agency of electricity we shall have 

 advanced still another step, and shall be enabled to devise other means 

 by which-a more perfect imitation of nature may be effected, or a more 

 complete control asserted over the phenomena of vegetable circulation. 



§ 4. Functions of the stem. 



The functions of the stem are probably as various as tbose of the 

 root, though the circumstances under which they are performed neces- 

 sarily involve these functions in considerable obscurity. 



The pith which forms the central part of the stem consists, as I have 

 already stated, of lubes disposed horizontally. When a coloured fluid 

 is permitted to enter the lower part of the stem in the experiments 

 above described, the pith remains untinctured in the centre of the col- 

 oured wood. Jt does not, therefore, serve for the conveyance of the sap. 

 Nor does it seem to be vitally necessary to the health and growth of the 

 plant, since Mr. Knight has shown that, from the interior of many trees, 

 it may be removed without apparent injures and in nature, as trees ad- 

 vance in age, it gradually diminishes in bulk, and in some species be- 

 comes apparently obliterated. 



The vessels of the wood, which surrounds the pith, perform proba- 

 bly both a mechanical and a chemical function. They serve to convey 

 upwards to the leaf the various substances which enter by the roots. 

 This is their mechanical function. But during its progress upwards, 

 the sap appears to undergo a series of changes. When it reaches the 

 leaves it is no longer in the slate in which it ascended from the root into 

 the stem. The difficulty of extracting the sap from the wood, at dif- 

 ferent heights, has prevented very rigorous experiments from being 

 made on its nature and contents at the several stages of its ascent. 

 These it is obvious must vary with the species and age of the plant, and 

 with the season of the year at which the experiment is made. But the 

 general result to be drawn from such observations as have hitherto been 

 made, is, that those substances which enter directly into the root, when 

 mingled with such as have already passed through the circulation of the 

 plant, undergo, during their ascent, a gradual jjreparation for that state 

 in which they become fit to minister to the growth of the plant. This 

 preparation is completed in a great measure in the leaf, though further 

 changes still go on as the sap descends through the bark. This deduc- 

 tion is strengthened by the fact that gaseous substances of various kinds 

 and in varying quantities exist in the interior of the wood of the grow- 

 ing plant. These gaseous subtances, according to Boucherie, are in 

 some cases equal in bulk to one-twentieih part of the entire trunk of the 

 tree in which they exist. They probably move upwards along with 

 the sap, and are more or less completely discharged into the atmosphere 

 through the pores of the leaves. That these gaseous substances not 

 only differ in quantity, but in kind also, with the age and species of 

 the tree, and with the season of ihe year, may, I think, be considered 

 as almost amounting to a proof that they have not been inhaled direct- 

 ly by the roots, but are the result of chemical decompositions which 



