TWINING OF TENDRILS. 781 



whose curvature is less than that of the support, provided the support is not too 

 slender nor the tendril too thick. 



The cases are very instructive, in reference to the pressure which the coils of 

 tendrils exercise on their supports, where leaves are embraced by strong tendrils, and 

 folded and compressed by them. 



What has now been said is merely intended to draw attention to the more important 

 mechanical principles which must be taken into account in the twining of tendrils. 

 The biology of climbing plants and of those furnished with tendrils, so fertile in extra- 

 ordinary adaptations, cannot be gone into in detail. On this subject the reader will 

 find in Darwin's treatise quoted above a mass of beautiful observations most admirably 

 described. 



Since the physiological function of tendrils is to take hold of supports (generally 

 other plants) in order to allow the slender-stemmed plant which is furnished with them 

 to climb up, the point of greatest importance is for the tendril to be brought into con- 

 tact with a support. This is usually effected with extraordinary perfection by the re- 

 volving nutation not only of the tendril itself but also of the apex of the shoot that bears 

 it at the time when it is sensitive, thus causing every object anywhere within reach of 

 the tendril which could be used as a support to be brought almost inevitably into 

 contact with it. The apex of the shoot which bears the tendril usually describes an 

 ascending elliptic helix, the revolution being completed in from one to five hours. As in 

 the case of twining stems, a strong positive heliotropism would be injurious, as it would 

 often carry the tendril away from the supports. Some tendrils appear in fact to be 

 not heliotropic (those of Pisum according to Darwin), in others a weak positive helio- 

 tropism is shown by the fact that the revolving nutation takes place more quickly 

 towards the light than away from it. Some tendrils, strikingly those of the Virginian 

 creeper and Bignonia capreolata, have the remarkable power of developing broad discs 

 at the end of their branches when they remain in contact for some time with hard 

 bodies, which attach themselves like cupping glasses to rough surfaces, and enable the 

 plant to climb up vertical walls when it finds no slender support round which it can coil. 

 In this case it is obviously necessary that the tendril should turn towards the wall which 

 serves as its support in order to become attached to it, and this is effected by negative 

 heliotropism, which causes the tendril to approach the wall shaded by foliage, where it 

 now performs its revolving movements of nutation — one might almost say its groping 

 movements— creeps along the surface, finds out the crevices and depressions, and 

 developes its adhesive discs. 



