96 CUKMIVAL FUNtTJO.NS CF THE BARK. 



§ 6. Functions of the bark. 



The inner bark being connected with the under layer of vessels in the 

 leaf, receives from tliem the sap after it has been changed by the action 

 of the air and light, and transmits it downwards to the root. 



The outer bark, especially in young twigs and in the stalks of the 

 grasses, so closely resembles the leaves in its appearance, that we can 

 have no difficulty in admitting that it must, not unfrequently, perform 

 similar functions. In the Cactus, the Stapelia, and other plants which 

 produce no true leaves, this outer bark seems to perform all the functions 

 which in other vegetable tribes are specially assigned to the abundant 

 foliage. During its descent through the inner bark, therefore, the sap 

 must in very many cases undergo chemical changes, more or less analo- 

 gous to those which usually take place in the leaf. 



It is by means of the inner bark that the stems of trees, such as 

 our forest and fruit trees, are enlarged by the deposition of annual 

 layers of new wood. The woody fibre is formed or prepared in 

 the leaf, and as the sap descends it is deposited beneath the inner sur- 

 face of the inner bark. It thus happens that, as the sap descends, it is 

 gradually deprived of the substances it held in solution when it left the 

 leaf, and in consequence it becomes difficult to say how much of the 

 change, which the sap is found to have undergone when it reaches the 

 root, is due to chemical transformations produced during its descent, and 

 how much to the deposition of the woody fibre and other matters it has 

 parted with by the way. 



Among other evidences of such changes really taking place during 

 the descent of the sap, I may mention an observation of Meyen [Jahres- 

 bericht, 1839, p. 27], made in the course of his experiments on the re- 

 production of the bark of trees. In these experiments he enclosed the 

 naked wood in strong glass tubes, and in three cases out of eight the 

 tubes were burst and shattered in pieces. This could only have arisen 

 from the disengagement of gaseous substances, the result of decomposi- 

 tion. While, therefore, such gases as enter by the roots or are evolved 

 in the vessels of the wood during the ascent of the sap, escape by the 

 leaf along with those which are disengaged in the leaf itself, it is proba- 

 ble that those which are produced as the result of changes in the bark, 

 descend with the downward sap, and are discharged by the root.* 



In the bark of the root it is probable that still further changes take 

 place — and of a kind which can only be eflecled during the absence of 

 light. This is rendered probable by the fact that the bark of the root 

 frequently contains substances which are not to be met with in any 

 other part of the plant. Thus from the bark^of the fresh root of the ap- 

 ple tree a substance named phloridzine, possessed of considerable medi- 

 cal virtues, may be readily extracted, though it does not exist in the 

 bark either of the stem or of the branches. 



In fine, as the food which is introduced into the stomachs of animals, 

 undergoes continual and successive chemical changes during its pro- 

 gress through the entire alimentary canal — so, numerous phenomena 

 indicate that the sap of plants is also subjected to unceasing transforma- 



* SproDg||teays that the steins and twigs, and the stalks of the grasses, all absorb oxygen 

 and give offCarbonic acid.— CAemte, II., p. 341. 



