4 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 50 



abnormally high sap pressures develop (Fig-. 3). Pressures up to 

 60 pounds per square inch have been reported. However, pres- 

 sures of only 5 to 10 pounds per square inch are common in many 

 wetwood-affected trees. The gas contains approximately 46 per 

 cent methane. 34 per cent nitrogen, 14 per cent carbon dioxide, 



5 per cent oxygen, and 1 per cent hydrogen. It does not contain 

 carbon monoxide or illuminants. 



Sap accumulates under pressure in the diseased wood and 

 produces the water-soaked condition that gives rise to the name 

 wetivood. The sap from this wood contains phosphorus and an 



Fij?. 2. — A section of elm trunk aSTected with wetwood shows dark 

 brown diseased areas in isolated portions of some annual growth rings (as 

 at arrow .4) and brown streaking in part of the current -season growth ring 

 (as at arrow li). 



