HABITS OF CLIMBING PLANTS. 175 



dril had the power of thus acting, it would strike against the 

 extremity of the shoot, and be arrested by it. As soon as all 

 these branches of the tendrils begin to stiffen themselves in 

 this remarkable manner, as if by a process of turgescence, and 

 to rise from an inclined into a vertical position, the revolving 

 movement becomes more rapid ; and as soon as the tendril 

 has succeeded in passing the extremity of the shoot, its re- 

 volving motion, coinciding with that from gravity, often 

 causes it to fall into its previous inclined position so quickly, 

 that the end of the tendril could distinctly be seen travelling 

 like the minute-hand of a gigantic clock." — (p. 75.) 



Cucurbitaceous tendrils are mostly compound, in this case 

 three-forked. When one of the lateral branches has firmly 

 clasped any object, the middle branch continues to revolve. 

 If a full-grosvn tendril fails to reach and lay hold of any 

 object, it soon ceases to revolve, bends downwards, and coils 

 up spirally from the apex. Indeed it often coils while still 

 outstretched and revolving, the tendency to shorten (as we 

 presume) on the inner side from the tip downward, which is 

 usually brought into action by contact with an extraneous 

 body, at length operating spontaneously. Uncaught tendrils 

 when they thus coil up throw themselves of course into a simple 

 helix or spire. One end being free, this is the simple and 

 necessary consequence of the relative shortening of the con- 

 cave side, sufficiently continued. 



In a caught tendril, the relative shortening of one side, 

 (through which the tip hooks round and fixes itself to the sup- 

 porting object), being propagated downwards, the whole now 

 throws itself into a spiral form, — with more or less prompti- 

 tude according to the species, — thus pulling the free portion 

 of the tendril-bearing shoot nearer to the support, and within 

 easier reach of the next tendril above. Both ends of the ten- 

 dril being fixed, and the winding round an axis (real or imag- 

 inary) necessarily involving or being a twist, it is certain that 

 the caught tendril cannot now coil into a simple spiral, but 

 that the spire will at least be double, a coil near one end of 

 the tendril in one direction requiring the other to twist in the 

 opposite direction, unless indeed it undergoes torsion. So, as 



