J * MR. DAKW1N ON OUMBIKQ PLANTS. 



elapsed from the first observation, that is until 17 days after the 

 tendril was fully grown. 



The best proof of the intimate connexion between the spiral 

 contraction of a tendril and the previous act of clasping a support, 

 is afforded by those tendrils which, when caught, invariably 

 contract into a spire, whilst as long as they remain unattached 

 they continue straight, though dependent, and thus wither and 

 drop off. The tendrils of Rignonia, which are modified leaves, 

 thus behave, as do the tendrils of the three genera of Vitacea?, 

 and these are modified flower-peduncles. The tendrils, however, of 

 JEccremocarpus, which is allied to Bignonia, contract spirally even 

 when they have caught nothing. The uneaught tendrils of the 

 Cardiospermum, and to a certain extent those of the Jlfutisia, roll 

 themselves up not into a spire, but into a helix. 



The spiral contraction which ensues after a tendril has caught 

 a support is of high service to all tendril-bearing plants ; hence 

 its almost universal occurrence with plants of widely different 

 orders. "When a shoot is inclined and its tendril has caught an 

 object above, the spiral contraction drags up the shoot. When 

 the shoot is upright, the growth of the internodes, subsequently 

 to the tendrils having seized some object above, would slacken the 

 stem were it not for the spiral contraction, which draws up the 

 internodes as they increase in length. Thus there is no waste of 

 growth, and the stretched stem ascends by the shortest course. 

 We have seen in the Cohcva, when a terminal branchlet of the 

 tendril has caught a stick, how well the spiral contraction of its 

 branches successively brings them one after the other into contact 

 with the stick, until the whole tendril has grasped it in an inex- 

 tricable knot. When a tendril has caught a yielding object, this 

 is sometimes enveloped and still further secured by the spiral 

 folds, as I have seen with Passijlora quadrangularis ; but this action 

 is of little importance. 



A far more important service rendered by the spiral contraction 

 is that the tendrils are thus made highly elastic. As was pre- 

 viously remarked under Ampelopsis, the strain is thus equally 

 distributed to the several attached branches of a branched tendril ; 

 and this must render the whole tendril far stronger, as branch 

 after branch cannot separately break. It is this elasticity which 

 saves both branched and simple tendrils from being torn away 

 during stormy weather. I have more than once gone on purpose 

 during a gale to watch a Bryony growing in an exposed hedge, 

 with its tendrils attached to the surrounding bushes ; and as the 



