TO GAIN A HIGHER TBMPERATUEE. 423 



training trees upon a horizontal plane ; the only effect of which 

 would be to expose a tree as much as possible to the effect of 

 that radiation which it is the very purpose of training to guard 

 against. 



The actual temperature to which a tree trained upon a wall 

 facing the sun is exposed is much higher than that of the 

 surrounding air, not only because it receives a larger amount 

 of the direct solar fays, but because of the heat received by the 

 surrounding earth, reflected from it and absorbed by the wall 

 itself. Under such circumstances the secretions of the plant 

 are more fully elaborated than in a more shady and colder 

 situation ; and, by aid of the greater heat and dryness in front 

 of a south wall, the period of maturity is much advanced. In 

 this way we succeed in procuring a Mediterranean or Persian 

 summer in these northern latitudes. When the excellence of 

 fruit depends upon its sweetness, the quality is exceedingly 

 improved by such an exposure to the sun ; for it is found that 

 the quantity of sugar elaborated in a fruit is obtained by an 

 alteration of the gummy, mucilaginous, and gelatinous matters 

 previously formed in it, and the quantity of those matters will 

 be in proportion to the amount of light to which the tree, if 

 healthy, has been exposed. Hence the greater sweetness of 

 Plums, Pears, &c., raised on walls from those grown on 

 standards. It has been already stated that an increase of heat 

 has been sought for on walls by blackening them ; and we are 

 assured in the Hortieultwal Transactions (iii. 330) that,, in the 

 cultivation of the Grape, this has been attended with the best 

 effects. But, unless when trees are young, the wall ought to 

 be covered with foliage during summer, and the blackened 

 surface would scarcely act ; and in the spring the expansion of 

 the flowers would be hastened by it, which is no advantage in 

 cold late springs, because of the greater liabihty of early 

 flowers to perish from cold. That a blackened surface does 

 produce a beneficial effect upon trees trained over it is, how- 

 ever, probable, although not by hastening the maturation of 

 fruit; it is by raising the temperature of the wall in autumn 

 when the leaves are falhng, and the darkened surface becomes 

 uncovered, that the advantages are perceived by a better com- 



