Page:Darwin - On the movements and habits of climbing plants.djvu/7

 twisted in an opposite direction to the other internodes on the same plant, and to the course of revolution; and this, according to Léon (p. 356), is the case with all the internodes of a variety of the Phaseolus multiflorus. Internodes which have become twisted round their own axes, if they have not ceased revolving, are still capable of twining, as I have several times observed.

Mohl has remarked (S. 111) that when a stem twines round a smooth cylindrical stick, it does not become twisted. Accordingly I allowed kidney-beans to run up stretched string, and up smooth rods of iron and glass, one-third of an inch in diameter, and they became twisted only in that degree which follows as a mechanical necessity from the spiral winding. The stems, on the other hand, which had ascended the ordinary rough sticks were all more or less and generally much twisted. The influence of the roughness of the support in causing axial twisting was well seen in the stems which had twined up the glass rods; for these were fixed in split sticks below, and were secured above to cross sticks, and the stems in passing these places became very much twisted. As soon as the stems which had ascended the iron rods reached the summit and became free, they also became twisted; and this apparently occurred more quickly during windy weather. Several other facts could be given, showing that the axial twisting stands in relation to inequalities in the support, and likewise to the shoot revolving freely without any support. Many plants, which are not twiners, become in some degree twisted round their own axes ; but this occurs so much more generally and strongly with training-plants than with other plants, that there must be some connexion between the capacity for twining and axial twisting. The most probable view, as it seems to me, is that the stem twists itself to gain rigidity (on the same principle that a much twisted rope is stiffer than a slackly twisted one), so as to be enabled either to pass over inequalities in its spiral ascent, or to carry its own weight when allowed to revolve freely.