338 THE PREMATURE DEATH OF THE BRANCHES OF WALL FRUIT-TREES. 



The pear-tree better retains its health and vigour, when trained to a 

 wall, than those of either of the preceding species, or than the cherry- 

 tree ; but the proper course of its sap is nevertheless greatly deranged ; 

 and it is difficult, and in some varieties almost impossible, to cause it 

 to flow properly to the extremities or nearly to the extremities of its 

 branches. Much the larger part of it is generally expended in the 

 production of what are called "foreright" useless shoots; and the 

 quantity of fruit which is afforded by the central parts of an old pear- 

 tree, when trained to a wall, is usually very small. 



The vine alone amongst fruit-trees appears capable of being bound 

 and trained to a great distance upon a wall without sustaining any 

 injury, its sap continuing to flow freely and abundantly to its very distant 

 branches. Owing to a peculiarity of structure and habit which is con- 

 fined to those species of trees from which nature has withheld the power 

 of supporting their own branches, the alburnum of all plants of this 

 habit is (as far as I have had opportunities of observing) excessively 

 light or porous ; and not being intended by nature to support its own 

 weight, or that of any part of the foliage of the tree, does not acquire 

 with age any increased solidity, like that of trees of a different habit ; and 

 on this account probably it never, how long soever deprived of exercise, 

 loses in any degree its power of transmitting the ascending sap. The 

 alburnum of those trees which nature has caused to support themselves 

 without external aid, becomes annually more firm and solid, and con- 

 sequently less well adapted to afford a passage to the ascending sap, and 

 as heart- wood it is totally impervious to that fluid. Whenever the 

 branches of such trees are wholly deprived of exercise, too rapid an 

 increase of the solidity of the alburnum probably takes place ; and it in 

 consequence ceases to be capable of properly executing its office. I have, 

 of course, never had an opportunity of examining the character of the 

 alburnum" of the Glycine sinensis, of which the garden of this Society 

 contains so splendid a tree ; but I do not entertain a shadow of a doubt 

 of its being extremely light and porous, like that of other trailing and 

 creeping plants which depend for support upon other bodies. 



