36 On the premature death of the branches of wall fruit-trees. 



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 confined to those species of trees, from which nature 

 lias withheld the power of supporting their own branches, the al- 

 burnum of all plants of this habit is (as far as I have had oppor- 

 tunities 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 al- 

 burnum of those trees, which nature has caused to support them- 

 selves, without external aid, becomes annually more firm and solid, 

 and consequently less well adapted to afford a passage to the as- 

 cending sap, and as heart wood it is totally impervious to that fluid. 

 Whenever the branches of such trees are wholly deprived of exer- 

 cise 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. 



