200 SHADE-TREES IN TOWNS AND CITIES 



The writer once had under observation a number of trees 

 suffering from gas-poisoning. There were elms, Norway 

 maples, sugar maples, white maples, and red maples among 

 them. About the time when the red and the white maples 

 began to show the usual symptoms of the poison, such as the 

 etiolation and the partial loss of the foliage, the sap began 

 to ooze out from a number of places on the trunk and main 

 branches. The sap did not come from openings made by 

 borers or other causes, but through splits of bark looking 

 normal on the outside. On removing the bark there was 

 seen a discoloration of the sap-wood, and the descending cur- 

 rent of sap flowed freely near the point of this discoloration. 

 If a part only of the bark covering this diseased spot was 

 cut away, the sap was seen coming out from under the por- 

 tion of the bark still covering the remainder of this blister- 

 like formation. The sap flowing from the splits in the bark 

 was frothy white in appearance (Plate 41, Fig. 3), and had a 

 fermented odor, as that of cider. 



The exact cause and nature of the formation of ulcers in 

 trees is not yet fully understood. The condition is variously 

 ascribed to an excess of humidity in the soil, to sudden 

 changes of temperature during the growing season, to the 

 non-utilization of all the sap for growth, to a decomposition 

 of internal tissues, and to a modification of the chemical com- 

 position of the sap. There is no doubt, however, that the 

 disease is due to a general disturbance of the nutrition of the 

 tree which is followed by local fermentation of bark areas. 



The division of Forest Pathology of the Bureau of Plant 

 Industry of the United States Department of Agriculture, 

 expects to undertake within a few years systematic work 

 on phenomena of the class to which ulcers and slime-flux 

 belong. 



