382 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM 



distal side (p. 97). I subjected the matter to the following 

 experimental test. 



Experiment 204.— My High Magnification Crescograph 

 enabled me to obtain a record of the normal rate of 

 growth of the stem of Thunbergia ; this was found to 

 be 0*7 [Jt per second. The stem was next subjected for 

 5 minutes to mechanical stimulation by friction applied to 

 all sides of the organ. The growth was now found to be 

 arrested, and it was not till after an hour that growth was 

 renewed. It is evident that continuous stimulation of one 

 side of the stem by friction against a rough support retards 

 or arrests the growth, with induced concavity, of the 

 stimulated side. The twining stem is therefore sensitive 

 to mechanical stimulation. 



The following may be taken as the sequence of events 

 which lead to twining. The autonomous torsion of the 

 stem gives rise to circumnutation of the overhanging part 

 of it, in the course of which movement the stem comes in 

 contact with a support. Continued autonomous torsion and 

 the contact-sensitiveness of the stem then co-operate to effect 



the twining. 



Having now indicated what is the essential mechanism 

 of twining, I may proceed to explain several sensitive 

 methods for the measurement of the torsional movement 

 and its induced variations. 



Method of Optical Magnification 



In order to study the effect of the change of anyone of the 

 external factors, it is essential to maintain all other factors 

 absolutely constant, a condition which can be secured only 

 for a short time. Hence necessity arises for rapid observa- 

 tion of the rate of normal torsion, and of the changes induced 

 in it by external agents. This I secured by the Optical 

 Method which I devised for my earher investigations on 

 autonomous torsion (' Plant Response,' 1906). The tip 

 of the plant is attached to a thin, torsionless string which 

 passes over a pulley with a suitable counterweight at the 

 other end ; the plant is thus held erect. A light concave 



