CHAPtVli.] WALL-FRUIT TREES. 199 
words, the subsoil. This is, however, false 
reasoning on true premises. It is quite true 
that the tree has ceased to bear in conse¬ 
quence of the descent of its roots; but the 
reason this descent is injurious is, that the 
ground far below the surface is cold, and 
frequently impregnated with stagnant water; 
and either that the roots thus become swollen 
and unable to perform their proper functions, 
in which case the leaves turn yellow, and 
the tree appears to wither, or that they 
supply the tree with an abundance of poor 
thin watery sap quite unsuitable for the pro¬ 
duction of fruit. On the contrary, when the 
roots are kept near the surface, though they 
have no air-vessels except in the spongioles, 
these spongioles imbibe air and carbonic acid 
gas from the atmosphere with all the moisture 
they take up; and thus the vessels are not 
only kept in a healthy state by not being 
overcharged with w^ater without air, but the 
sap is so thickened and enriched with the 
carbonic acid gas, that it is brought into a 
proper state for forming those deposits which 
lead to the production of fruit. 
The use of walls is to afford tender plants 
