196 CLIMBERS, PARASITES, SAPROPHYTES. 



spirally, the convex side being the lower side, but in a few 

 days it lengthens and becomes straightened out, at the same 

 time sweeping slowly round through the air. The move- 

 ments of the young tendril are "automatic," i.e. they are 

 not made in response to any external stimulus, and after its 

 vigorous growth has ceased the tendril begins again to grow 

 more rapidly on its upper side, so that the lower surface now 

 becomes concave. The tendril, if it has not met with any 

 support, keeps on coiling in this way (owing to the more 

 rapid growth of the upper side), and finally loses its sensitive- 

 ness when fully grown. 



If, on the other hand, the tendril has, in the first 

 period of its growth (the uncoiling stage), come into 

 contact with a solid support, after sweeping around and 

 " feeling " for one, it rubs against the support with its lower 

 surface and begins to coil round it, and after making a few 

 coils it becomes harder. The free part of the tendril, be- 

 tween the plant and the support, then becomes coiled and 

 for mechanical reasons there occurs at least one reversal of 

 the coil, sometimes several reversals ; see Art. 206 (c) . The 

 spiral coiling of the free intermediate part of the tendril is 

 due to excessive growth on the upper side, and is very 

 important, since it draws the plant up nearer the support 

 and also forms an elastic spring which enables the plant to 

 yield to the wind and acts as a buffer against shocks that 

 might tear the tendril from the support. 



In most cases, the sensitiveness of a tendril is confined to 

 the lower side and is greatest near the tip, though some 

 plants have tendrils which are sensitive on both upper and 

 lower sides. In any case, however, no coiling occurs if both 

 upper and lower side of the tendril be stimulated at the 

 same time. In shrubby climbers the tendrils undergo 

 secondary growth in thickness ; the tendrils of a shrub have 

 to act for several years, while those of a herbaceous plant 

 are only needed for a single season. To a certain extent the 

 strength of a tendril becomes ultimately proportional to the 

 weight it has to bear, so that if a number of the young 

 tendrils be removed the remaining ones become stronger, 

 provided they are still capable of growth. 



Every student of Botany should read Darwin's Climbing Plants 

 (Murray, 2s. 6d.). Very careful experiments made by later workers 



