244 FUNDAMENTALS OF FRUIT PRODUCTION 



In the United States, Lippincotti°" traced isotherms for combined June, July, 

 August and September temperatures and correlated them with the grapes growing 

 in the zones thus marked out. Here a combined selection is exercised by summer 

 and by winter temperatures and in addition, as pointed out elsewhere the summer 

 temperatures doubtless have some influence on the effect of winter cold. How- 

 ever, there can be no doubt that summer temperatures have a direct effect of 

 their own. In favored localities in the zone with a mean of 65°F. he found 

 Chnton and Delaware, with a few other varieties. In the 67°F. zone he included 

 Concord and Hartford Prolific; Isabella, Diana and Rogers' Hybrids he con- 

 sidered to require 70°F. Catawba, Norton's Virginia, Herbemont and Scupper- 

 nong were assigned to regions with average summer temperatures of 72° or higher. 



Differences within the Variety for Separate Processes. — Different 

 processes in the same plant have different optimum temperatures. 



Phytolacca decandra at Carmel, Cal., grows well but flowers only under certain 

 conditions as, for example, when prostrate branches receive sufficient additional 

 heat from the soil to enable them to form viable seeds, while the erect stems 

 do not.^" In connection with fruit setting it is shown that lower temperatures 

 than usual convert male blossoms of the papaya into perfect flowers. Schimper 

 points out the difference in the temperature curve for the two forms of gaseous 

 exhange and states that assimilation occurs at lower temperatures than any 

 other function. He cites evidence of assimilation in Abies excelsa and other 

 plants at — 40°C. and cites Bohm as finding the optimum for the walnut at 30°C. 

 No distinct respiration could be observed in Abies below — 10°C.; this function 

 increases, speaking in general terms, with the temperature until the lethal point is 

 approached. Quoting Schimper"^ again: "There are, however, certain physio- 

 logical processes for which not only the optima, but also the upper zeros are so 

 low that, as a rule, they can take place only in winter, late autumn, or early 

 spring. The category of functions that are active at low temperatures only 

 includes among others the obscure processes which are fermentative in nature, 

 according to Sachs' hypothesis, and which awaken into activity hibernating 

 parts of plants ; among such processes may be cited the conversion of starch into 

 fatty acids and the reverse. . . . Lower temperatures exert a favourable influ- 

 ence on the sexual organs and on the parts oecologically connected with them 

 (perianths, inflorescence axes) in many parts of the temperate and frigid zones. 

 The cardinal degrees for the growth — and perhaps for the inception — of the 

 primordia of flowers are often much lower than for the growth of vegetative 

 shoots, so that the former are favoured by a relatively lower temperature, and the 

 latter by a high temperature, during development. It is well known that 

 Crociis, Hyacinthus, and other perennial herbs do not send out flowers or inflores- 

 cences at a high temperature, but shoot out luxuriantly into leaf. Also in the 

 forcing of fruit trees the temperature must be kept moderate before, and espe- 

 cially during, the blossoming period. For the same reason many temperate 

 plants seldom blossom in the tropics; for example, most of our fruit trees. . . . 

 Kurz found in the mountains of Burmah that increased coolness due to increased 

 altitude expedited the blossoming of temperate plants such as Rhododendron and 

 Gentiana, but delayed that of tropical ones." 



