CHAP. VH.] WALL FRTIT TREES. 169 



quentlv 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, from the want of 

 air, they supply the tree with an abundance of 

 poor thin watery sap, quite unsuitable for the 

 production 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 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 7 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 nails is to afford tender plants 

 the heat necessary to mature their fruit, by 

 reflecting the sun's rays back upon it; and by 

 giving out to the fruit during the night the 

 heat they have absorbed during the day. They 

 are also useful in sheltering the plant from cold 

 winds; and in preventing the branches from 

 bruising each other in violent storms. This 

 being the use of walls, it is evident that only 

 those trees should be trained against them that 

 require protection ; and the south and south- 

 east walls being warmer than the others, it is 

 equally evident that the trees trained against 

 them should be only those that require a great 

 deal of heat to mature their fruits. There are 

 some fruits, such as the apple, which too much 



