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 un- 



