TWINING PLANTS: TENDRILS 297 



(192 1 ) has shown that the whole movement can be referred 

 to a succession of negative geotopic reactions in an organ 

 which responds vigorously with a strong over-curvature. 



As the stem apex twists and circles it twines round any 

 support which is nearly vertical — the precise angle of 

 inclination which prevents successful twining differs in 

 different plants — and which is not too thick. The rather 

 loose coils at first formed are tightened up by subsequent 

 straightening of the stem, a process in which gravity seems 

 to play the chief part, though contact stimulus may have 

 something to do with it. The grip of the twiner on its 

 support may be further improved by the maturing of 

 prickles or hooks, as in the hop. Only when twining is 

 complete do the leaves of any region of the stem expand 

 fully. As a rule the direction of twining is constant and 

 specific ; usually it is counter-clockwise, as in the scarlet 

 runner, less often clockwise, as in the hop. 



Tendrils.— Tendril climbers show the highest degree 

 of specialisation in making use of external support ; in 

 these one organ or another is modified as a tendril, the 

 slender, whip-like part which grasps the support. Tendrils 

 are characterised by extreme sensibility to contact with a 

 solid body. This may be admirably seen in the bryony. 

 If the tendril is once or twice lightly stroked with a match on 

 the lower side it may be seen to bend downwards at the 

 point of contact after the lapse of a minute or two. The 

 details of this reaction have been exhaustively studied, e.g. 

 by Darwin (18756) and Fitting (1903) ; a general account 

 is given by Jost. In few cases does stimulus to any side 

 of the tendril lead to bending in the direction of the stimulus ; 

 usually the reaction follows stimulation of the lower side 

 only. Curiously enough this is not due to lack of sensibility 

 of the other sides, for a tendril if stimulated equally above 

 and below does not react at all. Thus both sides are 

 sensitive, though only in the lower is stimulation followed 

 by the response of movement. Stimulation of the upper 

 side is perceived, indeed, but the reaction is only an inhibition 

 and not an independent movement. 



