160 OPEN AIR GRAPE CULTURE. 



" 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 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 maturity is much advanced. In this way we suc- 

 ceed in procuring a Mediterranean or Persian sum- 

 mer 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 gela- 

 tinous matters previously formed in it, and the quan- 

 tity 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, etc., 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 ' Horti- 

 cultural Transactions ' (HI. 330) that, in the cultiva- 



