172 LIFK: OlTMNKS OF GENERAL BIOLOGY 



the cover of the drnso foliage of adjacent i)lants. The palmetto in 

 dry open places lias i)ractically no above ground stem, "while in 

 moLst woods, plants of equal age have long slender trunks several 

 metres in height". So is it with many other j^lants m dark or 

 in moist places. 



We suggest then, that the shaded and humid conditions of the 

 forest may act as stimuli on individual plants generation after 

 generation, and that the jx^culiaritics, especially the more super- 

 ficial, are in pari moditicational, i.e. are imprcs.scd on each successive 

 generation. But this is only part of the answer, for there must, we 

 think, be added the suggestion that the shaded and moist environ- 

 ment has favoured those plants with an intrinsic tendency to grow 

 long shoots and to exaggerate mobility and sensitiveness. Ever\' 

 summer one may see in a deeply shaded wood the elimination of 

 unsuitable types whose seeds have been sown by birds or the wind. 



One step farther may be ventured. It may be suggested that the 

 environmental conditions of shade and humidity act as variation- 

 lilx'rating stimuli, and that they tend to evoke in the germ-cells 

 particular kinds of variations, such as those that find expression 

 in a twining habit. 



As for the detailed structures that assist in climbing, the tendrils 

 in particular, it is hardly possible at present to get beyond the 

 general Darwinian view that variations are of frequent occurrence 

 and that sjx^cial climbing organs have been derived from stems, 

 branches, flower-stalks, leaves, leaflets, petioles, midribs, and even 

 roots. The widespread occurrence of these adaptations is striking, 

 for, as Darsvin pointed out, more than half of the fifty-nine alliances 

 into which Lindley divided flowering plants include climbers of 

 .some kind or other. 



It is fitting that we should close this section by quoting the last 

 ]xiragra])h of Darwin's Climbing Plants. "It has often been vaguely 

 asserted that plants arc distinguished from animals by not having 

 the ]>ower of movement. It should rather be said that plants acquire 

 and display this power only when it is of some advantage to them; 

 this Ix'ing of comparatively rare occurrence, as they are affixed to 

 the ground, and food is brought to them by the air and rain. We see 

 how high in the scale of organisation a plant may ri.sc when we look 

 at one of the most ptTfect tendril-bearers. It first places the tendrils 

 ready for action, as a polypus places its tentacula. If the tendril b<^ 

 displaced, it is acted on by the force of gravity and rights itself. It 

 is acted on by the light, and Ixnds towards or from it, or disregards 

 it, whichever may be most advantageous. During several days the 

 tendrils or internodes, or both, spontaneously revolve with a steady 

 motion. The tendril strikes some object, and quickly curls round and 

 firmly grasps it. In the course^ of some hours it contracts into a spire, 

 dragging up the stem, and forming an excellent spring. All move- 



