242 FUNDAMENTALS OF FRUIT PRODUCTION 



winters are sufficiently mild. The grape is among the plants most 

 frequently cited by phenological workers as showing this same exaction 

 in its requirement. Variety adaptation in apples probably depends on 

 growing season temperature, among other factors. In addition it seems 

 rather likely that this factor is operative in another way though its 

 effects necessarily are masked by their own results; low summer tem- 

 perature may delay maturity to such an extent that an ensuing winter of 

 medium intensity is injurious. The obvious and immediate cause of 

 trouble here would be winter injury but the antecedent cause would be 

 the cool summer. Much of the winter injury characteristic of parts of 

 Europe seems to be involved with low summer temperatures. 



The effects of temperature alone in certain phases can be compared 

 best, perhaps, in plants of the deserts, since these regions show rather 

 greater uniformity in other conditions than most humid sections. In 

 the date palm temperature assumes considerable importance. 



According to Swingle i^^^ "The northern limit and the limit of altitude in 

 northwestern Africa at which dates can be grown are set more by the deficient 

 summer heat failing to ripen the fruit than by the cold in winter." Very early 

 ripening dates, he reports, can be grown far to the north where the summers are 

 not warm enough to ripen later varieties. Swingle confirms DeCandoUe's 

 calculation of 64.4°F. as the point below which no effect is produced on flowering 

 or fruiting of the date palm. Affirming that under desert conditions temperature 

 summations have considerable significance he states that 2000°C., using 18°C. 

 as the zero point, are necessary to ripen Deglet Noor dates satisfactorily. 



Mason^23 cites Caruso as authority for the statement that 51° to 52°F. 

 is zero point for the olive and adds that in California zero may be some- 

 what higher, probably 55° to 56°F. He assigns a definite number of heat 

 units as necessary for ripening the olive before autumn, but points out 

 that in some localities with low summer temperatures and little or no 

 frost in winter the fruit may remain longer on the trees. In some 

 places the requisite number of heat units is not accumulated until 

 December. 



The apple shows some indications of the effects of excessive summer 

 heat at some points in the United States and of deficient summer heat at 

 others. Along the southern limits of its successful culture there is a 

 general tendency to vigorous vegetative growth with httle fruit produc- 

 tion and much of the fruit that is borne rots on the tree. In that period 

 when apple varieties were being tested and when the varietal composition 

 of the orchard was not determined by market standards of the large 

 cities, the Ribston Pippin attained a much greater popularity in eastern 

 Maine than in any other section of the country then growing apples. 

 Other Enghsh varieties were more favorably received there than in any 

 other state. Downing^^ considered that the Ribston attained far better 



