TWINERS AND CLIMBERS 33 



in strength and size when attached, and function as most efficient attaching 

 organs, lasting in some cases for several seasons. Certain Bignonias have 

 twining stems in addition to tendrils or coiling leaf-stalks, while in a few 

 species shoots are even formed which develop attaching roots 1 . The 

 tendrils themselves may also not only coil but attach themselves at their 

 tips by special disks produced under the stimulus of contact 2 . Certain 

 plants only develop the climbing habit under special conditions, for instance, 

 most twiners lose the power of twining when the action of gravity is 

 eliminated, while Polygonum aviculare and Galium Mollugo develop 

 scrambling stems in moist shady rich localities, but short erect or creeping 

 stems in dry exposed situations. 



The climbing habit enables the plant to reach light and air without 

 spending a large amount of material in forming a stout erect stem. For 

 this reason their growth in length is especially rapid, the stem of a hop 

 for instance often becoming twelve metres in length during the summer. 

 The conducting tissues need to be especially well developed in the thin 

 stems of climbers. The wood-vessels for instance may be above one to 

 three metres in length, and over half a millimetre in diameter, in this 

 way the resistance to the unusually rapid flow of sap being reduced to 

 a minimum 3 . 



Twiners are specially adapted for climbing up single thin supports, 

 whereas tendril-climbers grow best when numerous points of attachment 

 are presented, as in bushes or hedges. Root-climbers again are adapted 

 to cling to rough erect surfaces such as walls, rocks or tree-trunks, which 

 is only possible to tendril-climbers possessing attaching disks such as 

 Ampelopsis hederacea, or claw-like grappling-hooks such as Bignonia unguis. 

 All climbers are not equally well adapted, and in general tendril -climbers 

 exhibit the most striking instances of special adaptation. 



The young stem of a climber usually attains a certain length before 

 climbing begins. Thus a seedling scarlet-runner develops at first a stout 

 erect stem with a pair of simple foliage leaves, and only after a period 

 of nutritive preparation produces the slender actively circumnutating twining 

 stem with its trifoliate foliage leaves. In all cases if the stem fails to reach 

 a support, it grows prostrate along the surface of the ground, until by 

 accident, aided by movements arising spontaneously or produced by the 

 wind, it comes into contact with a support. Climbers have no power 

 of seeking out supports, and even the negative heliotropism of the ivy 

 only comes into play when the stem is already quite near to the wall. 

 Circumnutation naturally aids the plant in finding a support, and it is 



1 Darwin, Climbing Plants, 1875, pp. 93, 101, 135. 



2 Darwin, I.e. ; Cohn, Bot. Ztg., 1878, p. 27. Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, 

 T. xv, p. 208 seq. 



3 Cf. Ewart, On the ascent of water in Trees, Phil. Trans., 1904, p. 65. [The longest vessel, 

 564 cms., was found in Wistaria chinensis.} 



PFEFFER. Ill D 



