TWINING OF TENDRILS. 775 



The direction of the twining is usually constant in the same species ; but it does 

 sometimes happen, as in Solanum Dulcamara and Loasa aiirantiaca, that different indi- 

 viduals twine in opposite directions. Darwin found, in these two species and in Scyphan- 

 thus elegans and Hibbertia dentata, that the same stem will sometimes twine first in one 

 and then in the other direction. 



The positive heliotropism of twining internodes is generally feeble ; a powerful 

 heliotropism would obviously be only a hindrance to the twining and especially to the 

 revolution, by which an effort, so to speak, is made to reach the support. Heliotropism 

 is however shown by the fact that when the light falls from one side only, revolution 

 takes place more quickly towards the source of light than away from it ; as e. g. in 

 Ipomecea jucunda, Lonicera brachypoda, Phaseolus, and Humulus. 



It may be concluded from what has been said on the mechanism of twining that 

 there is for every species a certain maximum of thickness of the support at which the 

 twining is possible. The support must not be much thicker than the diameter of the 

 coils which the shoot can make without a support ; if the support is too thick, the apex 

 of the shoot attempts to make coils by its side, and these eventually become effaced. 

 Darwin {I.e. p. 22) acknowledges his ignorance of the cause why the climbing plant 

 cannot twine round supports which are too thick ; de Vries's experiments however seem 

 to give a sufficient explanation. 



The movements of twining internodes are more energetic the more favourable the 

 external conditions of growth, and the more rapid the growth itself; they are therefore 

 vigorous when food is abundant, temperature high, and the plants contain abundance of 

 sap. The direct action of light is not necessary for twining, since even etiolated plants 

 (as Tpomcca purpurea and Phaseolus multijlorus cling closely to their support in the dark. 

 The assertion of Duchartre that Dioscorea Batatas) does not twine in the dark reduces 

 itself, according to de Vries's more recent observations, to the fact that while normal 

 green shoots climb more loosely in the dark, they cease rotating and twining when they 

 become etiolated. 



Sect. 25. The Twining of Tendrils ^ Under the term Tendril may be 

 comprised all filiform or at least slender long and narrow parts of plants which 

 possess the property of curving round slender solid supports with which they come 

 in contact during their growth, clinging to them in consequence, and thus at length 

 fixing the plant to them. Tendrils are therefore at once distinguished from climbing 

 internodes by their irritability to contact or pressure. 



Organs of the most various morphological description may assume this physi- 

 ological property. Sometimes tendrils are metamorphosed branches, as in Vitis 

 Ampelopsis, Passiflora, and Cardiospenmim Halkacabum, where they may be con- 

 sidered more accurately as metamorphosed flower-stalks or inflorescences. In 

 Cuscuta the whole stem may be regarded as a tendril rather than as a climbing 

 stem. In other cases, as in Clematis, Tropaeolum (Fig. 455) Maurandia, Lopho- 

 spermum, Solamim jasmwoides, &c., the petioles may serve as tendrils. In 

 Fumaria officinalis and Corydalis clavicidata the whole of the finely-divided leaf is 

 sensitive to contact, and its separate parts have the power of twining round slender 

 bodies. In Gloriosa Pla?itii and Flagellaria indica the mid-rib protruding beyond 

 the leaf serves as a tendril. In many Leguminosae and Bignoniacese and in Cobcea 



as the twining ; and therefore also the same as the spontaneous torsion and the revolving nutation 

 of the same plants. 



' See the literature quoted in the preceding section. 



