AUTONOMOUS TORSION (>67 



stimulus in general — so similarly with regard to geotropic 

 stimulus, there is no necessity for the assumption of any 

 specific dia-gcotropic sensibility. 



Autonomous torsion. — We have now seen how torsional 

 response is induced in an anisotropic organ. We have next 

 to deal with that interesting class of phenomena which 

 consists of the natural torsional movements of growing 

 organs. It is to be remembered that, in referring to these 

 torsional movements, we mean the torsional growth-move- 

 ments of certain stems themselves about their own axes. 

 The term positive torsion in such cases means a movement 

 which appears, when looked at from above, to take place in 

 the same direction as the movement of the hands of a watch, 

 the term negative being used in the contrary sense. 



Such torsional movements are found to occur in the stems 

 of climbing plants. In some of these they are of a positive, 

 and in others of a negative character, while in still others 

 again they alternate, the natural torsion being, say, positive 

 at one period, and negative at another. 



Now, we have seen in the case of leaves that, owing to 

 different rates of growth in the antagonistic upper and lower 

 halves of the petiole, autonomous epinastic or hyponastic 

 movements occur in a rectilinear direction. When the 

 growth of the upper half is predominant, the movement of 

 the organ is downwards, and vice versa. More complex, 

 however, is that case in which the line of maximum growth 

 is a spiral revolving about an axis, thus bringing about a 

 growth-movement which causes a torsion of the organ round 

 that axis. In the case of epinasty and hyponasty, it was 

 said that the rectilinear movement induced was due, not to a 

 total absence of growth on either side, but to the relatively 

 greater growth of one of the antagonistic halves. Similarly, 

 in these torsional movements we have to remember the 

 existence of antagonistic elements in the stem, and it is the 

 predominant growth of one of these, the right-handed or left- 

 handed, that brings about the resultant positive or negative 

 torsion. 



