^f^ 



CONTACT STIMULUS AND TWINING 381 



the stems of twining plants have an autonomous torsional 

 activity which is not dependent upon geotropism. This 

 will be clear when it is realised that it is differential growth 

 which causes the torsional movement, growth itself 

 being a phenomenon 

 of autonomous activity. 

 The stimulus of gravity, 

 it is true, modifies 

 growth, but does not 

 initiate it. Examina- 

 tion of the ribbed stem 

 of a twining plant before 

 it has bent over shows 

 that the erect stem 

 had undergone a twist 

 (fig. 216). Circumnuta- 

 tion of the bending stem 

 is thus principally due 

 to the torsional activity 

 of the organ. When the 

 circumnutating stem 

 encounters a more or 

 less vertical support, it 

 coils round it. 



V 



Contact-Stimulus 

 AND Twining 



Fig. 216. The twist of the ridges of the stem 

 (left figure) indicates the natural torsion of 

 the stem. The right figure exhibits shoot 

 IT- i_ twining round support in anti-clockwise 



Kohl lound that the direction (Thunbergia coccinea). 



stem of Calystegia 



twined round a loose string, the stem being concave at all 

 points of contact. In spite of this, there is a prevalent 

 opinion that twining stems are not sensitive to the stimulus 

 of mechanical contact, a view which raises difficulties in 

 explaining the primary cause of twining round a support. 

 In the case of tendrils which are highly sensitive to contact, 

 twining is produced by the retardation of growth at the 

 points of contact, aided by acceleration of growth of the 



