432 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 4 



Pressure changes of fractions of a pound, 

 probably induced largely by changes in 

 external air temperatures, were obtained 

 during February and March of 1943. 

 Pressures definitely higher than normal 

 became apparent in April, 1943, and in- 

 creased until early September, when they 



Although the mean maximum tempera- 

 ture for June falls within this range, 

 pressure readings are not then at the maxi- 

 mum, perhaps because of the limited 

 amounts of carbohydrates and other mate- 

 rials available at that time for fermentation 

 in the infected wood. Possibly at that time 



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MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 

 1942 1943 



Fig. 27. — The average monthly air temperature and pressure in two non-fluxing wetwood 

 elms in Hinsdale; record covers 22 months. The temperature and pressure curves are similar 

 in shape. However, the peak of the temperature curve occurred in July in each year, while the 

 peak of the pressure curve occurred in August in 1942 and in September in 1943. 



reached a peak, following which they 

 gradually declined, dropping to zero by 

 January, 1944. 



The relation between the pressure curve 

 and the air temperature curve for the 2 

 years is shown in fig. 27. The pressure 

 curve represents the average mean month- 

 ly pressure readings from two non-fluxing 

 elms in Hinsdale. In shape it is very simi- 

 lar to the mean temperature curve. How- 

 ever, the peaks of the pressure curve oc- 

 curred in August in 1942 and in Septem- 

 ber in 1943, while the peaks of the tem- 

 perature curve occurred in July in both 

 years. The similarity of these two curves 

 indicates that the period of the year when 

 the wetwood organism is most active is 

 in general the period of the year during 

 which high temperatures occur — July, 

 August and early September. The period 

 during which maximum fermentation 

 occurs in the trees, as indicated by maxi- 

 mum pressure readings, is mainly in 

 August when the mean maximum air tem- 

 perature — 22 to 30 degrees C. or 71.6 to 

 86.0 degrees F. — coincides with the opti- 

 mum temperature for the organism : 24 

 to 30 degrees C, or 75.2 to 86.0 degrees F. 



much of the food of the tree is being used 

 in wood growth. According to Priestly 

 (1930), Beilmann (1935) and Mac- 

 Dougal (1936, 1938), xylem growth in 

 several species of trees is most rapid during 

 late April, May and June. MacDougal 

 (1938) reported that cambial activity of 

 elm ceases about August 1 in New York 

 state. 



A pressure of 60 pounds per square 

 inch, recorded for one elm tree in Hins- 

 dale on June 20, 1942, is quite remarkable 

 when it is compared with pressures record- 

 ed by MacDougal (1932) and Beilmann 

 (1940) for healthy trees of various 

 species. MacDougal found internal maxi- 

 mum trunk pressures of 20 mm. of mer- 

 cury and Beilmann 18 mm. of water. 

 MacDougal & Working (1933, page 85), 

 who studied gas pressures of the pneu- 

 matic system of trees, reported positive 

 pressures of not more than 20 mm. of 

 mercury (approximately 0.3868 pound) 

 ". . . and never more than a few milli- 

 meters less than barometric." The elm in 

 Hinsdale which developed 60 pounds of 

 pressure in 1942 had wilted during 1941. 

 A gaugecock was installed in this tree on 



