August, 1945 



Carter: Wetwood of Elms 



435 



A.M. The minimum pressure occurred be- 

 tween 10:00 P.M. on August 3 and 12:10 

 A.M. on August 4 and between 7 :05 and 

 8:00 P.M. on August 12. These observa- 

 tions indicate that the diurnal pressure 



maximum pressures and maximum tem- 

 peratures occurred most often at 1 :00 p.m. 

 These data show, as did the temperature 

 and pressure curves for August 3 and 12, 

 that pressure reaches its maximum at 



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AIR TEMPERATURE 



._ HEALTHY ELM 



INTERNAL TRUNK TEMPERATURE 



HEALTHY ELM 



WETWOOD-AFFECTED ELM 



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Fig. 30. — Air temperatures, internal trunk temperatures in a healthy tree, and pressures 

 in a healthy tree and in a wetwood-atfected tree, all three trees greenhouse-grown; recorded 

 on November 16, 1943. The trunk temperature curve and the pressure curve for the healthy 

 trees are similar in shape to the air temperature curve. A rapid drop in air temperature 

 between 1:00 and 1:10 p.m. did not cause a similar drop in pressure in the wetwood-affected 

 tree. Pressure in the diseased tree did not decrease until the air temperature dropped below 

 68.5 degrees F. Zero represents atmospheric pressure. 



cycle in wetwood-affected trees is not in- 

 fluenced by changes in air temperature, as 

 long as the air temperature is in the opti- 

 mum range for the organism. However, 

 air temperatures above and below the op- 

 timum range for the organism apparently 

 influence the rate of fermentation and thus 

 affect pressure. 



Pressure and air temperature readings 

 were taken at 15-, 30- or 60-minute inter- 

 vals on 17 days between August 2 and 

 September 1 and on 15 days between 

 September 29 and November 16, 1943. 

 For the 17 days, maximum pressures oc- 

 curred most often at 11:00 a.m., while 

 maximum temperatures occurred most 

 often at 1 :00 p.m. For the 15 days, both 



about 11 :00 a.m., when the maximum air 

 temperature is above the optimum tem- 

 perature range for the wetwood organism. 

 However, when the maximum air tem- 

 perature is not above the optimum tem- 

 perature range of the organism, pressure 

 and temperature both reach their maxima 

 at about 1 :00 p.m. The pressure cycle does 

 not reach a second peak during any one 

 day, regardless of the variations in air 

 temperature. This experiment indicates, as 

 did the previous one, that the diurnal 

 pressure cycle is not influenced by changes 

 in air temperature so long as the air tem- 

 perature is in the optimum range for the 

 organism. 



The relationship between air tempera- 



