24 NATURAL HISTORY READER. 



simple twiner. And again, a twiner from being best fitted 

 to climb bare stems often has to start in the shade, whereas 

 a leaf- or tendril-climber can ramble for the whole extent of 

 its growth up the sunn)' side of a bush. 



18. To repeat once more the steps which it is believed 

 have occurred in the evolution of climbing plants : It is 

 probable that plants have become twiners by exaggerating 

 a swinging-round or revolving movement, which occurred 

 in a rudimentary form, and in a useless condition, in some 

 of their ancestors. This movement has been utilized for 

 twining, the, stimulus which has driven the process of 

 change in this direction having been the necessity for 

 light 



19. The second stage has been the development of sen- 

 sitive leaves by a twining plant. No doubt at first no leaf- 

 climber depended entirely on its leaves — it was merely a 

 twiner which helped itself by its leaves. Gradually the 

 leaves became more perfect, and then the plant could leave 

 oft' the wasteful plaa of growing spirally up a stick, and 

 adopt the more economical and more effective one of pure 

 leaf-climbing. 



20. Finally, from sensitive leaves were developed the 

 marvelously perfect tendrils which can perceive one fiftieth 

 of a grain, and can show distinct curvature within twenty- 

 five seconds after being touched, tendrils with delicate, 

 sticky ends, or endowed with the power of moving toward 

 the dark, or of creeping into little cracks, or with that 

 mysterious sense of touch by which a tendril can distin- 

 guish a brother tendril from an ordinary twig, and can dis- 

 tinguish the weight of a rain-drop banging to it from that 

 of a bit of thread— in short, all the delicate contrivances 

 which place tendril-bearers so eminently at the head of the 

 climbing plants. 



Francis Darwin. 



