266 MODIFIED CIKCUMNtlTATION. Chap. V. 



in an almost vertical plane, and therefore describe very 

 narrow ellipses, whereas the many kinds of tendrils 

 which consist of metamorphosed leaves, make much 

 broader ellipses or nearly circular figures; and thus 

 they have a far better chance of catching hold of a 

 support on any side. The movements of climbing 

 plants have also been modified in some few other 

 special ways. Thus the circumnutating stems of Sol- 

 nanum dulcamara can twine round a support only 

 when this is as thin and flexible as a string or thread. 

 The twining stems of several British plants cannot 

 twine round a support when it is more than a few 

 inches in thickness ; whilst in tropical forests some 

 can embrace thick trunks ;* and this great difference 

 in power depends on some unknown difference in 

 their manner of circumnutation. The most remarkable 

 special modification of this movement which we have 

 observed is in the tendrils of Eehinocystis lobata ; these 

 are usually inclined at about 45° above the horizon, 

 but they stiffen and straighten themselves so as to 

 stand upright in a part of their circular course, namely, 

 when they approach and have to pass over the summit 

 of the shoot from which they arise. If they had not 

 possessed and exercised this curious po^er, they would 

 infallibly have struck against the summit of the shoot 

 and been arrested in their course. As soon as one of 

 tliese tendrils with its three branches begins to stiffen 

 itself and rise up vertically, the revolving motion 

 becomes more rapid; and as soon as it has passed 

 over the point of difSeulty, its rcotion coinciding 

 with that from its own weight, causes it to fall into its 

 previously inclined position so quickly, that the apex 

 can be seen travelling like the hand of a gigantic clock 



'The Move]uents and Habits of Climbing Plants,' p. 36. 



