32 MOVEMENTS OF CURVATURE 



increase, and the same would also occur if one side only performed an 

 active contraction or expansion, the other being passively stretched or 

 compressed l . Pfeffer has shown that in the latter case the force of 

 expansion in an active half of the pulvinus of Trifolinm would lie between 

 0-6 to 2 atmospheres. 



PART II 



TWINERS AND CLIMBERS 



SECTION 8. General. 



According to the mode of climbing we may distinguish between 



(a) twiners, like the hop, which wind their slender stems around supports, 



(b) tendril-climbers which use special coiling attaching organs for this 

 purpose, (c) root-climbers which attach themselves by means of aerial 

 roots, (d) scramblers, like the bramble or Goosegrass which support them- 

 selves by means of the asperities or hooks upon the stem, or by the 

 unfolding of the leaves after the stem has grown through a bush- No 

 hard and fast distinction can be drawn between the different groups, how- 

 ever, since the tendrils of the Virginian creeper, for instance, attach themselves 

 by means of sucking-disks, while the attaching roots of Vanilla are able 

 to coil around supports. Typical root-climbers are Hcdera helix, Ficiis 

 stipulata, and Tecoma, which are able to attach themselves to walls or 

 to the trunks of trees. 



In the case of scramblers no phenomena of special physiological 

 interest are shown, the stem grows upwards in virtue of its heliotropism 2 

 so long as it receives support, while the unsupported ends trail downwards 

 owing to their own weight. The stems of root-climbers are negatively 

 heliotropic and negatively geotropic, so that they strive to grow erect 

 but avoid light, hence pressing themselves against walls or the trunks of 

 trees. In this way the required conditions for the formation of aerial 

 attaching roots are produced, namely shade, moisture, and possibly contact 

 also in many cases. 



The twining stems of Cuscuta are not only irritable to contact like 

 tendrils, but also form parasitic roots, while the twining stems of Hoya 

 develop attaching roots. Many hook-climbers possess hooks which grow 



1 [The rigidity depends upon the magnitude of the opposing forces, and therefore is ultimately 

 dependent upon the hydrostatic pressure within the cells. An ' active ' contraction produced by 

 a fall of the osmotic pressure allowing the stretched cell-walls to contract on the ' active ' side, will 

 allow the cell-walls on the ' passive ' convex side to be more expanded by their internal pressure. 

 The increase of volume involving an absorption of water results in a fall of osmotic pressure on this 

 side also. Hence the antagonizing forces decrease on both sides, and the rigidity does not increase 

 but diminishes.] 



2 Schenck, 1. c., pp. 7, 134, 156 ; Darwin, I.e. 



