few other special ways.  Thus the circumnutating stems of Solnanum 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 Echinocystis lobata; these are usually inclined at about 45o 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 or the shoot from which they arise.  If they had not possessed and exercised this curious power, they would infallibly have struck against the summit of the shoot and been arrested in their course.  As soon as one of these 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 difficulty, its motion 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 Movements and Habits of Climbing Plants,’ p. 36. [page 267]

A large number of ordinary leaves and leaflets and a few flower-peduncles are provided with pulvini; but this is not the case with a single tendril at present known.  The cause of this difference probably lies in the fact, that the chief service of a pulvinus is to prolong the movement of the part thus provided after growth has ceased; and as tendrils or other climbing-organs are of use only whilst the plant is increasing in height or growing, a pulvinus which served to prolong their movements would be useless.

It was shown in the last chapter that the stolons or runners of certain plants circumnutate largely, and that this movement apparently aids them in finding a passage between the crowded stems of adjoining plants.  If it could be proved that their movements had been modified and increased for this special purpose, they ought to have been included in the present chapter; but as the amplitude of their revolutions is not so conspicuously different from that of ordinary plants, as in the case of climbers, we have no evidence on this head.  We encounter the same doubt in the case of some plants which bury their pods in the ground.  This burying process is certainly favoured by the circumnutation of the flower-peduncle; but we do not know whether it has been increased for this special purpose.

Epinasty—­hyponasty.

The term epinasty is used by De Vries* to express greater longitudinal growth along the upper than

* ‘Arbeiten des Bot.  Inst., in Würzburg,’ Heft ii. 1872, p. 223.  De Vries has slightly modified (p. 252) the meaning of the above two terms as first used by Schimper, and they have been adopted in this sense by Sachs. [page 268]