208 HOW PLANTS CLIMB. 



adheres firmly, although dead, to the wall and to its own stem. 

 Such tendrils will remain like this for 5, 10, or 15 years ! 

 Darwin found that a single disc-bearing branch would bear a 

 strain of two pounds ; a whole tendril, usually carrying five 

 branches, would therefore endure a strain of ten pounds ! 



Of the Passion-flower I can only say that Passiflora gracilis 

 was found by Darwin to exceed all other climbing plants in 

 rapidity of action, and all tendril-bearers in the sensitiveness of its 

 tendrils. 



4. — Tendrils which are uiodified Stipides. 



I simply name one case — Sniilax aspera — where this occurs. 

 Their position places the matter beyond doubt. As they grow 

 they diverge from each other, and are thus enabled to clasp an 

 object behind the stem. They avoid the light, and do not 

 spirally contract. Neither they nor their internodes revolve. 

 Sniilax is in all respects an imperfect climber. There are no 

 tendrils in the young state ; the stem is zigzagged and furnished 

 with spines, growing only to some eight feet high. The reason of 

 the existence of these tendrils is not easy to explain. Darwin 

 regards it as a kind of degraded relic of a genus formerly possess- 

 ing highly organised tendrils, seeing that even now some species 

 have much longer ones than S. aspera. 



A few isolated and brief remarks on tendril-life as a whole 

 must close our somewhat rambling study of this class. In most 

 tendril-bearers the young internodes revolve in ellipses, varying in 

 rate from one to five hours, a smaller range than that of twiners. 

 Twining power is almost ;///, but the revolving motion serves to 

 aid the tendrils in finding support. Tendrils themselves revolve 

 spontaneously in most cases, sometimes with the internodes, 

 sometimes at slower speed ; some do not revolve — e.g.^ Lathyrns^ 

 as we saw just now. In one case — the A^irginia Creeper— neither 

 internodes nor tendrils revolve. 



Tendrils revolve by curving of the whole length, except the 

 base and tip. The movement is due to unequal growth travelling 

 round the tendril and bowing it, a process we saw before in the 

 twining stem. To this cause is due not only revolution, but 

 movement to and from the light, and spiral contraction. 



Darwin thinks that motion following touch in tendrils is due to 



