TWINING PLANTS COMPARED WITH TENDRILS. 67.5 



that a shoot which has been growing for some time without a support, on being 

 afforded opportunity to twine round a support obtains after a few days a new 

 lease of life, so to speak, and grows much more actively. Here, then, we have an 

 effect similar to that met with in the case of tendril-plants, where likewise the whole 

 plant attains greater vigour when it can employ its tendrils. 



With regard to the question as to the irritability of twining plants being a cause 

 of twining itself, the fact must moreover be mentioned that by no means all twining 

 plants agree -in this respect. It was long ago made known by ]\Iohl that the 

 Dodder [Cuscufa) is impelled by mere continued contact with a support to twine 

 closely around it, exactly like a tendril ; these plants, however, are at the same 

 time slightly geotropic, and as they twine they thus tend to ascend, in which 

 they resemble twining plants. A few more words, again, as to the leaf-stalks of 

 Lygodium ; these, as is usually said, behave exactly like twining shoot-axes, and 

 in fact exhibit the greatest possible similarity to them ; yet INIohl with equal right 

 designates them as tendrils. According to my observations, in fact, the leaf-stalks 

 of Lygodhwi act at the same time both as tendrils and twining plants : tendrils in so 

 far as they are induced to twine round a support, exactly like true tendrils, only by 

 continued contact ; while they resemble twining plants in that they run round the 

 stem only upwards, though in doing this they can, like Bhimcnhachia, alter the direction 

 of the spirals. 



The opposite extreme is afforded by the peduncles of the female inflorescence of 

 the water-plant VaUisneria, which are often more than a metre long and about as thick 

 as sewing-thread. At the period when fertilization takes place these long filaments are 

 extended in order that the female flowers may float on the surface of the water ; 

 after fertilization, however, the filament contracts in close spirals like a cork-screw, 

 evidently because the one side shortens or the other lengthens, just as in the case 

 of the coiling up of tendrils or erect twining shoots which have met with no 

 support. 



Unfortunately there is not space here to render clearer the question as to the 

 true nature of twining shoot-axes, though I must still mention one or two of the 

 more important facts. 



It has already been stated that the upper end of a spirally wound twining 

 stem makes flat and nearly horizontal turns, whereas the older turns, further 

 removed from the bud, are steeper; and the same is the case when no support is 

 present. In other words, the turns are at first flat and become higher and steeper with 

 increasing age, especially when the support is thin. It is scarcely to be questioned 

 that this change is due to the influence of gravitation ; in any case it is of use to the 

 plant, since when the coils which at first lie only loosely on the support raise 

 themselves and tend to become straighter, they must at the same time go on clasping 

 the support more closely. To make this clear, it is only necessary to wind a flexible 

 caoutchouc tube round a rod in loose low coils, and then extend the two ends of the 

 tube with the hands ; the coils will then become steeper and apply themselves more 

 closely to the rod. But even young coils begin to exert pressure on the support; if 

 the latter is smooth and is then withdrawn from the coils, they becoine closer, exactly 

 as in the case of coiling tendrils. 



I stated above that the numerous torsions observe.! on many twining shoots, 



x x 2 



