668 LECTURE XXXVIII. 



those of Ampelopsis hederacea (the Virginian Creeper), and Bignoin'a capreolata, have 

 the remarkable property of developing broad cushions of tissue at the apices of their 

 branches, when they remain for some time in contact with hard bodies : these 

 cushions apply themselves to rough surfaces like suckers, and thus make it possible 

 for the plants mentioned to climb up vertical walls, when they find no thin supports 

 around which to twine. In this case it is evidently important that the tendrils should 

 turn towards the wall serving as a support, in order to fix themselves to it, and 

 this is attained by means of negative heliotropism, which drives the tendrils towards 

 the wall shaded by the foliage, where they then, in virtue of their nutations, make 

 various, one might almost say groping, movements, and glide over the surface, dipping 

 especially into depressions and cracks, and then develope their attaching discs. 



I now pass on to the twining plants, which have already been shortly characterised 

 at the commencement of this lecture. In the first place stress is once more to be laid on 

 the fact that we are here concerned not with special climbing organs springing from 

 the shoot-axis, but with the shoot-axis itself which supports the foliage-leaves and 

 flowers, and which is at the same time adapted for climbing up supports. The 

 function of a twining shoot-axis is to twine round an upright support in the direction 

 of a spiral line, and to apply itself so close to it that, by means of the mutual 

 friction it clings sufficiently fast to the support, not to slide off from it again even 

 under the weight of the appendages ; the latter event does happen, indeed, even 

 with the best of twining plants, if the surface of their vertical support — a rod for 

 instance — is too smooth to furnish a strong mutual friction. This explains why most 

 twining shoot-axes tend themselves to have very rough surfaces, provided by means 

 of torsion-ridges and furrows, or curved, hook-like, silicified hairs, &c. : never- 

 theless, there are also twining plants the shoot-axis of which is perfectly smooth, 

 e. g. Bowiea volubilis. 



One of the chief differences which distinguish twining shoots from tendrils, must 

 be understood from the beginning, and clearly borne in mind ; namely, that 

 twining plants only coil themselves round and climb up upright supports. This 

 distinguishes them at once from tendrils, which are able to coil themselves round 

 horizontal as well as upright supports, and downwards as well as upwards. It 

 appears to be best that the upright support of a twining plant should stand quite 

 vertical, though it is not impossible for these plants to twine round obliquely upright 

 supports : it appears that these may be regarded as still not unfavourably situated 

 even when the angle with the horizon is as much as 45°. The majority of twining 

 plants, however, are no longer able to twine actively if their support forms a 

 smaller angle with the horizon, although it cannot be denied that, under peculiar 

 circumstances, some climbing plants are able to make a few turns round even hori- 

 zontal supports. In our further considerations we shall always assume for the sake 

 of simplicity that we have to do with nearly vertical supports. 



A further point, conspicuous even on superficially regarding the matter, lies in 

 the fact that twining shoot-axes wind themselves round the support in a definite 

 direction, according to the species of plant in each case. The Hop, the Honeysuckle 

 {Lonicera capri/olium), and a few less well known plants such as Tatnus elephaiitipcs , 

 Polygonum scandens, &c. twine to the right, as we are in the habit of saying — i. e. from 

 the right below to the left above as one looks at the support ; but the majority of 



