HOW PLANTS CLIMB. 209 



contraction of cells on the concave side ; a point on which he 

 differs from Sachs, who attributes this motion, as wtII as all 

 others, to the unequal growth spoken of. 



Tendrils, when revolving, manage, in a way very wonderful 

 to see, to avoid clasping the stem to which they belong (Gray). 



All tendrils are sensitive, the degree, of course, varying. 

 They curve towards the side touched. Usually they are not 

 sensitive to the touch of other tendrils^ or of water-drops. Has 

 the latter fact anything to do with their relation to showers of 

 rain ? 



Some tendrils are retarded in their movement by light, others 

 quickened ; others, as those of the Pea, not influenced at all. In 

 some the invariable bending from light to dark is as certain as 

 that of a vane from the wind. 



Tendrils contract spirally when their ends are caught by any 

 object. This shortens them, and renders them elastic. This 

 spiral contraction is almost without exception ; it may ensue in 

 the branches only, as in the Pea ; in most cases, the base does 

 not contract. 



It is due to unequal growth. It is independent of revolving 

 motion, and not necessarily related to the act of clasping, since 

 many tendrils unattached perform this act either as a helix or as 

 a spire ( Passiflora). In this case there is only one spire formed, 

 but in attached tendrils the spire is always double and reversed, 

 with a straight part between the two spires. There is, of course, 

 a simple physical reason for this, into which I cannot now enter 

 more fully. (" Climbing Plants," pp. i66 — 169.) 



For a summary of the use and service of contraction, I refer 

 the reader to Darwin's own words ("Climbing Plants," pp. 163, 164). 



III. — Hook-Cliaibers. 



Examples are seen in Galium aparine^ Brambles, and some 

 Roses. There is no revolving power, the plants climbing solely by 

 the hooks. Smilax and Hop, belonging to former classes, have hooks. 



IV. — Root-Climbers. 



Of these. Ivy is a very good type. Rhiis^ or Poison Ivy, is 

 another. Fiais repe?is, a species of Fig, emits drops of viscid 

 fluid to assist its upward progress. Cusciita (Dodder) has root- 

 like suckers used for a similar purpose. 



Sutton^ Suri'ey ; July^ 1886. 



