WOUNDS, LESIONS AND MECHANICAL INJURIES 33 



sponse to the tensions produced by such curvatures, as may be 

 seen from tests made in which the tension of two sides of a 

 straight root are thrown out of equilibrium. 1 It is a direct reaction 

 to forces which change the customary form of the organ. 2 



A B 



Fig. 1 6. Root-systems of Pisum sativum: A, developed in soil and curved geo- 

 tropically. B, grown in water-culture, automatically curved. Secondary roots origi- 

 nating from convexities. After Noll. 



48. Wounds, Lesions, and General Mechanical Injuries. In- 

 tense mechanical forces which cut, tear, or crush the protoplasts 

 or their membranes, exert a stimulating effect upon the neighbor- 

 ing uninjured elements as well as the entire organism in certain 

 instances. The most immediate result of such stimulation con- 



1 Noll, F. Ueber den bestimmenden Einfluss von Wurzelkriimmungen auf der 

 Enstehung und Anordnung der Seitenwurzeln. Bonn, 1900. 



2 MacDougal. Curvature of roots. Bot. Gazette, 23 : 344. 1898. 



