FUNCTIONS OF THE STF.M AND LEAVES. 89 



have taken place on the stem itself, as the sap mounted upwards to- 

 vrards !he leaves. 



We have seen that the roots exercise a kind of discriminating power 

 in admitting to the circulation of the plant the various substances whicli 

 are present in the soil. The vessels of the stem exhibit an analogous 

 power of admitting or rejecting the solutions of diflerent substances into 

 which they may be immersed. Thus Boucherie states that, when the 

 trunks of several trees of the same species are cut ofF above the roots, 

 and the lower extremities immediately plunged into solutions of differ- 

 ent substances, some of these solutions will quickly ascend into and pen- 

 etrate the entire substance of the tree immersed in them, while others 

 will not be admitted at all, or with extreme slowness only, by the ves- 

 sels of the stems to which they are respectively presented. On the 

 other hand, that which is rejected by one species will be readily admit- 

 ted by another. Whether this partial stoppage of, or total refusal to ad- 

 mit, certain substances, be a mere contractile effort on the part of the 

 vessels, or be the result of a chemical change by which their exclusion 

 is eftected or resisted, does not as yet clearly appear. That it does not 

 depend upon the lightness and porosity of the wood, as might be sup- 

 posed, is shown by the observation that the poplar is less easily pene- 

 trated in this way than the beech, and the willow than the pear tree, 

 the maple, or the plane. 



These various functions of the woody part of the stem are performed 

 chiefly by the newer wood or alburnum, or, as it is often called, the sap 

 wood of the tree. As the heart wood becomes older, the tubes of which 

 it consists are either gradually stopped up by the deposition of solid 

 substances which have entered by the roots, or by the formation of 

 chemical compounds, which, like concretions in the bodies of animals, 

 slowly increase in size till the vessels become entirely closed — or they 

 are by degrees compressed laterally by the-growth of wood around them, 

 so as to become incapable of transmitting the ascending fluids. Per- 

 haps the result is in most cases due in part to both these causes. This 

 more or less perfect stoppage of the oldest vessels is one reason why the 

 course of the sap is chiefly directed through the newer tubes.* 



The functions of the bark, which forms the exterior portion of the 

 stem, will be more advantageously described, after we shrill have con- 

 sidered the purposes served by the leaves. 



§ 5. Functions of the leaves. 



The vessels of which the sap w(5od is composed extend upwards into 

 the fibres of the leaf. Through these vessels the sap ascends, and from 

 their extremities diffuses itself over the surface of the leaf. Here it un- 

 dergoes important chemical changes, the extent, if not the exact nature, 

 of which will appear from a short description of the functions which the 

 leaves are known or are believed to discharge. 



1°. When the roots of a Vtv'mg plant are immersed in water, it is a 



• As the newest roots are prolongations of the newest wood, it may be supposed that the 

 fact of these roots being the chief absorbents from the soil, is a sufficient reason why that 

 which is absorbed by them should also pass up through the wood with which they are most 

 closely connected. But that the pores of the heart wood are really incapable of transmit- 

 ting fluids, is shown by plunging the newly cut stem of a tree into a coloured solution — the 

 newer wood will be dyed, while more or less of the central portion will remain unchanged. 



