(Edema of the Tomato. Ill 



In the absence of careful data covering a period of several weeks 

 there could not be made any satisfactory comparison with the difference 

 between soil and air temperature out doors during the growing months. 

 However, it will be interesting to note such differences in this connec- 

 tion. For this reason there is appended here a table of soil and air 

 temperatures adapted from data published by President Fernald of the 

 Maine Agr, Exp. Station.* 



The difference in the mean temperature of the soil (from 1-36 inches) 

 and air for May, June and July is thus seen to be approximately 10° 

 Fahr. Comparing the temperature of the soil from 1-12 inches, it is 

 found to be from 5°-6° Fahr. below the mean temperature of the air. 

 It is doubtful, however, if these temperatures taken only at the stated 

 times of day, really give the information desired in relation to the 

 bearing of soil and air temperatures on root pressure and transpiration. 

 For through the day when the temperature is the highest that of the 

 air is still higher and with the aid of light and agitation of the air 

 transpiration is sufficient to relieve the plant. The early period of the 

 night is the time when out doors the conditions for root pressure as 

 indicated by temperature would be more favorable than they would for 

 transpiration. But terrestrial radiation soon lowers the temperature of 

 the soil so that the plants exist under this disadvantage a comparatively 

 short time. 



Here again it is necessary to bear in mind the very short nights 

 during which transpiration is low in comparison with the long days 

 when it is very active. The light conditions are reversed in the forcing 

 house and frequently the light is very feeble with little agitation of the 

 air while there is a continued maintenance of soil temperatures suitable 

 for active root absorption. Thus while it can not be asserted positively 

 that the temperatures of the forcing house are such as to add to the 

 complex disturbance of the physiological processes, thei'e are indications 

 that such is the case. 



Since turgescence is mainly dependent upon root absorption for the 

 supply of water and a turgid condition of the plant soon follows 

 strong root pressure unless transpiration is taking place rapidly, it has 

 become customary to say that root pressure causes turgidity. Of 

 course it is understood that this is not the immediate cause. Water 

 cannot be pressed into a cell and cause turgescence. The cell wall is 

 permeable and as rapidly as water could be driven in at one side it 



*Annual Report, Part IV., Maine State College Agr. Exp. Station, pp. 

 155-174. 



