440 BOTANICAL GAZETTE [December 



regarded as due to internal forces. It is evident that these forces 

 must reside in the primary tissues, while the final stability of the 

 internode is due to the development of secondary, lignified tissues, 

 corresponding with the last part of the rigidity curve. Apparently 

 lignification stops these internal forces and "sets" the stem with 

 whatever torsion it has acquired. It is very likely that if non- 

 twining stems were studied in a similar way with reference to 

 rigidity similar results would be obtained. 



I have not gone into morphological details because I believe 

 that the histology of twining plants with special reference to the 

 points mentioned deserves a more careful study than it has been 

 possible to give it thus far. Further experiments are necessary 

 before any more general conclusions can be drawn. 



Summary 



The scarlet runner bean and the black bindweed have been 

 studied with respect to torsion in a modified form of auxanometer. 

 It is demonstrated that, as the internode grows in length, at first 

 it twists but Httle, later it twists much more rapidly, and at the end 

 of its growth in length there is a slight reverse twist. Rigidity, or 

 "resistance to twisting," increases only slowly until near the end 

 of growth in length, when there is a sudden and extensive increase. 

 This final increase in rigidity accompanies the development of 

 secondary, lignified tissues. If we prevent the frame holding the 

 upper part of the internode from rotating, the lower part executes 

 homodromous torsion, and by overpowering the upper part pro- 

 duces antidromous torsion in it. The first internode of the bean 

 twists only a little when held in the apparatus. In a limited por- 

 tion of the vine when free to twist, the amount of torsion per unit 

 length is about constant, but this is not true of the vine as a whole,* 



Traverse City, Mich. 



