118 HORTICULTURE FOR SCHOOLS 



191. Cold. Minimum winter temperatures and the late- 

 ness and severity of spring frosts are of such importance that 

 they frequently govern the selection of the fruits to be grown. 

 Frost control is possible only where the frosts are light and 

 of short duration. Frost in spring injures the young growth, 

 and when it occurs at blooming time and during the early 

 setting of fruit, it may cause the loss of the entire crop. Still 

 nights are conducive to frost, for if there is no wind the coldest 

 air, being heaviest, settles down to the ground and so comes 

 in contact with the vegetation. For the same reason, frosts 

 are most severe on lowlands and in depressions, and lightest 

 on slopes. 



192. Moisture. A certain amount of water is necessary 

 for plant growth, and a large quantity is required for the 

 " sizing up" and maturing of fruit. Old trees require much 

 more moisture than young ones. It frequently happens that 

 trees when young will thrive on the rainfall of a region, in 

 which, when they become older, they will suffer from drought. 

 Depth and character of soil affect the amount of moisture 

 necessary. In loose soils of coarse texture, more rainfall is 

 needed than is the case with soils of finer texture, which are 

 more retentive of moisture. In humid countries it is impor- 

 tant that the rainfall occur to a sufficient extent during the 

 growing season. In semi-arid regions the time and distribu- 

 tion as well as the amount of the rainfall are important. For 

 example, an amount of rain which would be sufficient if it 

 fell during the fruiting season might be inadequate if dis- 

 tributed throughout the year. Irrigation makes it possible 

 to grow fruit in very dry regions. 



193. Air moisture. Air is said to be saturated when it 

 contains all the moisture it is capable of holding. Warm air 

 can hold more moisture than cold air. Air blowing over large 

 bodies of water or moist regions tends to collect moisture, 

 while air moving over arid regions loses moisture. Humid 

 air is favorable to the growth of most plants, but encourages 



