iv Movement and Nervous Action in Plants 67 



the guiding stimulus being the attraction of gravity ; climb- 

 ing plants bend towards the light by a movement closely 

 analogous to the incurvation which causes them to revolve ; 

 the spontaneous revolving movement is independent of 

 any outward stimulus, but is contingent on the youth of 

 the part and on vigorous health, which again depends on 

 a proper temperature and other favourable conditions of 

 life ; tendrils, and the petioles or tips of the leaves of leaf- 

 climbers, and apparently certain roots, all have the power 

 of movement when touched, and bend quickly towards the 

 touched side ; tendrils contract spirally soon after clasping 

 a support, but not after a mere temporary curvature (due 

 to pressure which is not permanent), and they ultimately 

 contract spirally if they have not come into contact with 

 any object. 



Interpretation of Movements. But the student natur- 

 ally asks what interpretation Darwin put upon these move- 

 ments of climbing plants. It will be easier to answer this 

 after we have considered what he thought of the many 

 other movements which plants exhibit. Meantime, how- 

 ever, a partial answer may be given. 



In the first place, it is plain that the climbing habit is a 

 useful one, such as would tend to persist in nature. " The 

 advantage gained by climbing is to reach the light and free 

 air with as little expenditure of organic matter as pos- 

 sible." 



In the second place, the habit of climbing is not an 

 occasional freak ; it is of widespread occurrence among 

 plants. Of the fifty-nine alliances into which Lindley 

 divided flowering plants, thirty-five, according to Darwin, 

 include twiners, leaf-climbers, or tendril-bearers. More- 

 over, the most different organs stems, branches, flower- 

 stalks, petioles, midribs of the leaf and leaflets, and 

 apparently aerial roots all possess this power. Not on- 



