76 STRUCTURE OF THE STEMS, ROOTS, AND LEAVES OF PLANTS. 



are all intended to contain liquid and gaseous substances, and to convey 

 them in a vertical, and sometimes in a horizontal, direction. The 

 tubes which compose the wood and bark are arranged vertically, as may 

 readily be seen on examining a piece of wood even wiili the naked eye, 

 and are intended to convey the sap upwards to the leaves and down- 

 wards to the roots. Those of which the pith and medullary plates con- 

 sist are arranged horizontal!}'-, and appear to be intended to maintain a 

 lateral intercourse between the pith and the bark — perhaps even to place 

 the heart of the tree within the influence of the external air. 



The root, though prior in its origin to the stem, may nevertheless for 

 the purpose of illustration be considered as its downward and lateral 

 prolongation into the earth — as the branches are its upward prolonga- 

 tion into the air.* When they leave the lower part of the trunk of the 

 tree, they differ little in their internal structure from the stem itself. 

 As they taper off, however, first the heart wood, then ihe i)ith, gradual- 

 ly disappear, till, towards their extremities, they consist only of a soft 

 central woody part and its covering of soft bark. These are connected 

 with, or are respectively prolongations of, the new wood and bark of the 

 trunk and branches. At the extreme points of the roots the bark be- 

 comes white, soft, spongy, and full of pores and vessels. It is by these 

 spongy extremities only, or chiefly, that liquid and gaseous substances 

 are capable either of entering into, or of making their escape from, the 

 interior of the root. 



The branches and twigs are extensions of the trunk ; and of the 

 former, the leaves may be considered as a still further extension. The 

 fibres of the leaf are minute ramifications of the woody matter of the 

 twigs, are connected through them with the wood of the branches and 

 stems, and from this wood receive the sap which they contain. The 

 green part of the leaf may be considered as a special expansion of tlie 

 bark, by which it is fitted to act upon the air, in ihe same way as the 

 spongy mass into which the bark is changed at the extremity of the root, 

 is fitted to act upon the water and other substances it meets with in the 

 soil. For as the fibres of the leaf are connected with the wood of the 

 stem, so the green part of the leaf is connected with its bark, and from 

 this green part the sap first begins to descend towards the root. 



§ 2. The functions of the root. 

 The position in which the roots of plants in their natural state are ge- 

 nerally placed, has hitherto prevented their functions from being so ac- 

 curately investigated as those of the leaves and of the stem. While, 

 therefore, the main purposes they are intended to serve are universally 



' The correctness of this comparison is proved by the fact that, in many trees, the branch 

 if planted will become a root, and the root, if exposed to the air, will gradually be trans- 

 formed into a branch. The banana in the forest, and the cirrant trpe in our gardens, are 

 familiar instances of trees spontaneously plantina their branches, and cnusing them to per- 

 form the functions of roots. In like manner, " if the stem of a youns plum or cherry-tree, 

 or of a willow, be bent in the autumn so that one-half of the top can be laid in the earth and 

 one-halfof the root be at the same time taken carefully up— shelterrd at finst and after- 

 wards gradually exposed to the cold— and if in tlie following year the remaining part of the 

 top and root be treated in the same way, the branches of tlie top will become roots, and the 

 ramifications of the roots will become branches, producing leaves, flowers, and fruit in due 

 BediSoa.— [l-oyiAow's ETwydopadia of Agricult.ure.'\ The tree is thus reversed in position, 

 and the roots and branches being thus mutually convertible cannot be materially unlike in 

 general structure. 



