CLIMBING PLANTS. 649 



the security of a tendril-bearer in a heavy wind with the ease with 

 which a twiner is partly blown from its support. Again, by looking 

 at those leaf -climbing plants which still possess in addition the power 

 of twining, it will be seen how incomparably better they grasp a stick 

 than does a 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 

 sunny side of a bush. 



We can thus see plainly how it has been an advantage for twining 

 plants to develop into leaf -climbers. We shall also find reasons why a 

 leaf -climber should find it advantageous to become a tendril-climber. 



We have seen how tendrils form a more sensitive, efficient grasping 

 organ than simple leaves. Tendrils possess also the valuable power of 

 shortening themselves by spirally contracting, and thus pulling up after 

 them the stem on which they grow, and afterward serving as springs 

 and breaking the force of the wind. We have had some cases where 

 we see the close relationship between leaf- and tendril-climbers, and 

 where we can see intermediate stages in the process of transition from 

 one method of climbing to the other. 



In certain kinds of Fumaria we can follow the whole process. Thus 

 we have one kind, which is a pure leaf-climber, grasping by its leaf- 

 stalks, which bear leaflets not at all reduced in size. A second genus 

 has the end leaflets very much smaller than the rest. A third kind 

 has the leaflets reduced to microscopical dimensions ; and, lastly, a 

 fourth kind has true and perfect tendrils. If we could see the ances- 

 tors of this last kind we should undoubtedly have a series of forms 

 connecting it with an extinct leaf -climber, resembling the series which 

 at present connects it with its contemporary leaf-climbing relatives. 



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 move- 

 ment, which occurred in a rudimentary form, and in a useless condi- 

 tion, 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. 



The second stage has been the development of sensitive 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 off the wasteful plan of growing spirally up a stick, and adopt 

 the more economical and more effective one of pure leaf -climbing. 



Finally, from sensitive leaves were developed the marvelously per- 

 fect tendrils which can perceive one fiftieth of a grain, and can show 

 distinct curvature within twenty-five seconds after being touched, ten- 

 drils, with delicate, sticky ends, or endowed with the power of mov- 

 ing toward the dark, or of creeping into little cracks, or with that mys- 



