258 APPLICATION OF PKINCIPLES. 



than that of the surrounding air, not only because it 

 receives a larger amount of the direct solar rays, 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 matu- 

 rity 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 ex- 

 ceedingly 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 (page 138) that an increase of heat has been 

 sought for on walls by blackening them ; and we are 

 assured in the Horticultural 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 dur- 

 ing 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 liability of 



