1919] Coit-Uodgson: Abnormal Shedding of TTashinfjton Navel Orange 325 



organisms. Thus Howard''" has shown witli wheat that at the genni- 

 nating period a fall of 10° to 12"^ P from 84° to 72° may mean the 

 difference between success and failure in obtaining a stand, since the 

 growth rate is almost inhibited at the former temperature. On the 

 other hand. Cannon"" lias shown that the maxiinuni rate temperature 

 for the mesquite, Prosopis velufina, and Opuntia is about 93° P. 

 Tobacco is another plant which thrives in hot soils. Leitch"' has shown 

 that for the garden pea, Pisum sativum, 85° P is the maximum rate 

 temperature and above 110° F no gi-owth whatever occurs. Appar- 

 ently, as in tiie ea.se of the aeration factor, no general rule for these 

 cardinal temperatures can be laid down. They must be determined 

 for each plant separately. Since growtli conditions absorption we 

 are justified in assuming that the cardinal temperatures for growth 

 are approximately those for absorption. 



The genus Citrus, as mentioned elsewhere, is native to the trojjies, 

 where it grew in the shade of other trees. Under the.se conditions the 

 soil was damp and soil temperatures certainly not high. It therefore 

 seems logical to as.sume that the temperatures favorable for root growth 

 in Citrus are not very high. As grown under clean cultivation in the 

 arid .southwest we believe that tlie absorbing roots are subjected 

 during a certain portion of the day to temperatures above the optinnim 

 and that during such period.s absorption is actually reduced. 



TABLE 11 

 Son. Temper.\tures (F.) at Edison, June 7, 1916 



A.M. P.M. 



Uour 9:1.5 10:15 11:15 12:15 2:15 3:15 4:15 5:15 



Six-inch dust imilcli 80.6 84.2 88.2 92.3 94.1 96.0 99..5 99.0 



Fir-st 6 inches 77.0 78.3 80.0 84.2 89.6 88.8 88.6 87.0 



Second 6 inches 77.0 76.1 76.1 78.0 82.4 82.4 82.4 80.6 



Third 6 inches 76.1 7.").0 75.0 75.3 79.2 77.2 78.3 78.0 



Fourth 6 inches 74.3 74.3 74.6 74.6 77.2 76.6 77.0 77.0 



Six-inch dust mulch in 



shade of tree 71.6 73.6 74.3 81.0 83.7 82..') 82.2 82.2 



To obtain an idea of the .soil temperatures prevailing in the 

 upper two feet of soil in 1916, a comparatively cool season, we made 

 a series of hourly readings at six-inch intervals. These may be found 

 summarized in table 11. This table shows tiiat during the afternoon 



ii" Influence of Weather on YieM of Wheat, Agr. Jour. India, vol. 2 (1916), 

 part 4. 



"" IJplation of the Rate of Root (Jrowth in Soedlings of Pronopis velutina to 

 the Temperature of the Soil, I'lant World, vol. 20 (1917), ]ip. 320-33. 



"i Some Experiments on the Influence of Temperature on the Rate of Growth 

 in Pi.sjim saliiitm, Ann. Bot., vol. 30 (1916), pp. 25-46. 



