6 MR. DARWIN 0>' CLIMBIXG PLAXTS. 



twisted in an opposite direction to the other internodes on the 

 same plant, and to the course of revolution ; and this, according 

 to L6on (p. 35G), is the case with all the internodes of a variety 

 of the Phaseolus multijlonts. Internodes which havo become 

 twisted round their own axes, it' they have not ceased revolving, 

 are still capable of twining, as I have several times observed. 



Mohl has remarked (S. Ill,) 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 steins 

 which had twined up the glass rods; for these were fixed in split 

 sticks below, and were secured above to cross sticks, and the steins 

 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 ap- 

 parently 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 f. 



* Professor Asa Gray has remarked to me, in a letter, that in Thuja occiden- 

 talis the twisting of the bark is very conspicuous. The twist is generally to 

 the right of the observer ; but, in noticing about a hundred trunks, four or flvo 

 were observed to be twisted in an opposite direction. 



t It is -well known that stems of many plants occasionally become Spirally 

 twisted in a monstrous manner ; and since the reading of this paper, Dr. 

 Maxwell Masters has remarked to me in a letter that "some of these cases, if 

 not all, are dependent upon some obstacle or resistance to their upward growth." 

 This conclusion agrees with, and perhaps explains, the normal axial twisting of 

 twining-plants ; but does not preclude the twisting being of service to the plant 

 and giving greater rigidity to the stem. 



