286 



GENERAL PLANT PATHOLOGY 



upon them and this unusual sight was stopped by a snow storm, which 

 followed on Sunday morning. The trees were loaded to the breaking 

 point. During the continuance of the storm, small branches were 

 taken off thirteen trees and shrubs and a blade of grass growing in West 

 PhUadelphia, and the thickness of ice upon them measured with a 

 pair of compasses. The accompanying figures drawn life size show the 

 relative thickness of the load of ice borne by the twigs, whose thickness 

 is shown in the drawings (Fig. 113). 



Fig. 114.— Happy white elm, Ulmus americana, plentifully supplied with ground 

 water near the surface in a depression of the glacial outwash plain at Westbury 

 L. I., July, 1915. 



The faU of hail stones may, if they are large enough, cause the 

 decortication of twigs, or the abrasion of other plant parts, thus per- 

 mitting the entrance of destructive bacteria and fungi to the interior 

 of the plants. 



Wind is an active agent in the breaking off of buds and limbs and 

 the formation of dangerous wounds. In such situations, as high moun- 

 tains, sand dunes and rocky shores, where trees are. exposed to the 

 forcible action of the wind, they assume a windswept, bisected, or 

 prostrate form, which is characteristic and picturesque (Fig. 16). 



