GROWING SEASON TEMPERATURES 249 



be different for different activities. Consequently it becomes extremely 

 difficult, if not impossible, to assign definite values to different temperatures 

 in their total growth effects, and the "heat units" necessary for com- 

 pleting certain changes, or carrying the plant through certain aspects 

 of its seasonal life history, vary considerabl}^ with conditions. In general 

 fewer heat units are required by a given plant in northern than in southern 

 latitudes. Other conditions being equally favorable, there is the best 

 varietal adaptation in sections where growing season temperatures 

 most nearly approach the optimum for the variety in question. The 

 importance of summer growing temperatures in determining the com- 

 mercial limits of fruit varieties is underestimated. Summer temperature 

 likewise exerts an important influence in determining the season of 

 maturity of the fruit. Soil temperature is of possible importance in 

 influencing growth and in determining the geographical range of certain 

 varieties. Injurious effects of soil temperatures can be minimized 

 sometimes by the use of stocks of the right kinds. Summer tempera- 

 tures also have an important indirect effect on orchard plants through 

 their influence on the range or activity of certain parasites. 



