HABITS OF CLIMBING PLANTS. 179 



spiral twiners, with leaf-climbers and tendril-bearers, which 

 agree in their power of spontaneously revolving and of grasp- 

 ing objects which they reach, are the most numerous in kinds, 

 and most perfect in mechanism ; they can easily pass from 

 branch to branch, and securely ramble over a wide and sun- 

 lit surface. 



After adducing some considerations in support of his 

 opinion that both leaf-climbers and tendril-bearers " were 

 primordially twiners, that is, are the descendants of plants 

 having this power and habit," Mr. Darwin asks : " Why 

 have nearly all the plants in so many aboriginally twining 

 groups been converted into leaf-climbers or tendril-bearers ? 

 Of what advantage could this have been to them ? Why did 

 they not remain simple twiners ? We can see several rea- 

 sons. It might be an advantage to a plant to acquire a 

 thicker stem, with short internocles, bearing many or large 

 leaves ; and such stems as are ill fitted for twining. Any 

 one who will look during windy weather at twining plants 

 will see that they are easily blown from their support ; not 

 so with tendril-bearers or leaf-climbers, for they quickly and 

 firmly grasj) their support by a much more efficient kind of 

 movement. In those plants which still twine, but at the same 

 time possess tendrils or sensitive petioles, as some species of 

 Bignonia, Clematis, and Tropaeolum, we can readily observe 

 how incomparably more securely they grasp an upright stick 

 than do simple twiners. From possessing the power of move- 

 ment on contact, a tendril can be made very long and thin ; so 

 that little organic matter is expended in their development, 

 and yet a wide circle is swept. Tendril-bearers can, from 

 their first growth, ascend along the outer branches of any 

 neighboring bush, and thus always keep in the full light ; 

 twiners, on the contrary, are best fitted to ascend bare stems, 

 and generally have to start in the shade. . . . 



" The object of all climbing plants is to reach the light and 

 free air with as little expenditure of organic matter as pos- 

 sible ; now, with spirally-ascending plants the stem is much 

 longer than is absolutely necessary •, for instance, I measured 

 the stem of a Kidney-bean which had ascended exactly two 



