1919] Coit-Eodgson : Abnormal Shedding of Washington Navel Orange 325 



organisms. Thus Howard 59 has shown with wheat that at the germi- 

 nating period a fall of 10° to 12° F 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 60 lias shown that the maximum rate temperature 

 for the mesquite, Prosopis velutina, and Opuntia is about 93° F. 

 Tobacco is another plant which thrives in hot soils. Leitch 61 has shown 

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

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

 ently, as in the case of the aeration factor, no general rule for these 

 cardinal temperatures can be laid down. They must be determined 

 for each plant separately. Since growth 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 tropics, 

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

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

 seems logical to assume 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 the absorbing roots are subjected 

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

 and that during such periods absorption is actually reduced. 



TABLE 11 

 Soil Temperatures (F.) at Edisox, June 7, 1916 



A.M. P.M. 



Hour 9:15 10:15 11:15 12:15 2:15 3:15 4:15 5:15 



Six-inch dust mulch 80.6 84.2 88.2 92.3 94.1 96.0 99.5 99.0 



First 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 75.0 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 S1.0 83.7 82.5 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 lie found 

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



59 Influence of Weather on Yield of Wheat, Agr. Jour. India, vol. 2 (1916), 

 part 4. 



r >° Relation of the Kate of Root Growth in Seedlings of Prosopis velutina to 

 the Temperature of the Soil, Plant World, vol. 20 (1917), pp. 320-33. 



6i Some Experiments on the Influence of Temperature on the Rate of Growth 

 in Pisum sativum, Ann. Bot., vol. 30 (1916), pp. 25-46. 



