Hints on Pruning. 437 



thing else in nature, must have its elements so adjusted that 

 combination between them and the air may take place regu- 

 larly and rapidly, when they are brought into action, that 

 properly organized wood may be formed. When this is not 

 the case, and certain elements are present in excess, decom- 

 position takes place in the redundant secretions; the decay of 

 the parts, and the ultimate death of the tree, is the conse- 

 quence. 



If we consider the various parts of the stem of a tree, viz., 

 the pith, the wood, the medullary rays, and the bark, to con- 

 sist of bundles of small, hollow tubes, of various sizes, and of 

 different kinds, intended to contain liquid and gaseous sub- 

 stances, and to convey them to different parts of the tree ; 

 moreover, if we consider the root as the lateral prolongation 

 of the stem downwards into the earth, and the branches as 

 the upward prolongation of the stem into the air, which cir- 

 cumstance is, in some measure, proved by the facility with 

 which branches may be transformed into roots, and roots 

 into branches, the original position of a tree may thus be re- 

 versed, and the roots and branches, being thus mutually con- 

 vertible, must, in their general structure, be very much alike. 

 Lastly, if we consider the leaves and spongioles as performing 

 analogous functions, and acting reciprocally upon each other, 

 which is evident from the appearance of trees in a hot day, 

 the supply of moisture inhaled by the roots being too small 

 for the quantity exhaled by the leaves, at the extreme points 

 of the roots, the bark becomes white, soft and spongy, a per- 

 fect mass of mouths, or pores, and vessels. It is by these 

 spongy extremities that liquid or gaseous elements are capa- 

 ble of entering into, or making their escape from, the interior 

 of the root. The branches and twigs, in like manner, are ex- 

 tensions of the trunk, and the leaves may be considered as a 

 still further extension. The fibres of the leaves are minute 

 ramifications of the woody matter composing the shoots, and 

 are by them connected with the larger branches, and through 

 them receive the sap which they contain. The green part of 

 the leaves may be considered as special expansions of the 

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

 way as the spongy terminations of the roots are fitted to act 

 upon the water, and other substances they meet with in the 



