ON THE OKIGIN AND PECULIARITIES OF CLIMBING PLANTS. 143 



come into contact with any object terminate in smooth shining discs. 

 These, however, after adhering to any object, sometimes become consider- 

 ably enlarged," * just as do those of Ampeloj^sis Veitchii. 



There is yet another genus which behaves in the same way. A leaf of 

 Arthrophyllum cirrhatum (Bignoniacece) has three leaflets, but the terminal 

 one is converted into a rod with a disc at the end. This is present in 

 the youngest bud long before it can be used for climbing purposes. 



The cucurbitaceous genus Trichosanthes supplies another instance of 

 the formation of adhesive discs in response to contact. T. ancjuina was 

 grown in a frame ; and although its tendrils are constructed for climbing, 

 like those of other members of this family, whenever their apices touched 

 the brick walls of the lights they formed adhesive pads, as in the above- 

 named plants. Lastly a common sea- weed, Plocaynium coccineum, with 

 a much branching thallus, produces adhesive pads at the tips of its 

 branchlets when they happen to rest on the thallus of another sea-weed, 

 such as of Laminaria. 



The anatomical structure of the stems of climbing plants is generally 

 very anomalous, but more or less characteristic of each family. Certain 

 common features prevail ; there is a relative increase cf pith and medullary 

 rays, the latter sometimes occurring as wedges. The number and size of 

 the water- conducting vessels are increased in comparison with ordinary 

 erect stems. In seme the pericycular fibres are much developed ; and as 

 all external layers of bark exfoliate, they form a "stringy" coat to the 

 stem, as occurs in Clematis and the vine. 



On the other hand the wood degenerates, this being a supportive tissue : 

 this means of support is superseded by suspension. This is seen in a 

 piece taken from the inner part of the stem of a vine, which will when 

 fresh be quite flexible and of a soft spongy texture. 



Such diflerences are easily perceptible in comparing a free-growing 

 shoot of ivy with that of a twig of the same diameter when supported by 

 climbing roots ; the relative differences between the wood and pith are 

 chiefly observable. Similar diflerences will be found in Wistaria, woody 

 nightshade, &c. They show that the change of structure occurs 

 simultaneously and immediately with the change of position, i.e. whether 

 the shoot is self-supporting or supported by some other means. 



It has often been noticed that as soon as some supporting contrivance, 

 hook, tendril, petiole, &c. has secured a hold on some structure these 

 organs at once increase in strength and put on mechanical tissues, as 

 wood &c. — for the tension caused by the w^eight of the plant is now 

 " felt " — and it at once sets to work to meet and resist the strain. t 



* Climbing Plants, p. 102, note. 



t I have discussed this more fully in The Origin of Plant St) ndures, chap, x, p. 197. 



