TORSION. 86 1 



The examples of torsion we have been considering so far are produced solely 

 Jby internal causes; the direction in which the striae run round the axis is usually 

 ;onstant in the same species ; but other instances of torsion frequently occur which 

 jsult from external and accidental circumstances. It is evident that when any 

 weight is attached to the side of an organ growing in a horizontal or oblique 

 irection, such as an internode, leaf, or tendril, the tendency will be to produce a 

 twisting of the organ round its axis. If the organ which is twisted in this manner is 

 ^ery elastic, the torsion will disappear when the weight is removed ; but if it is only 

 ?ery imperfectly elastic, the torsion will remain permanently, as in a twisted thread 

 ►f wax ; and this will be the case if the organ is in a growing state. This does 

 in fact occur in growing internodes, petioles, the mid-ribs of leaves, &c. If an 

 organ of this kind is fixed horizontally in wet sand, after a pin slightly weighted on 

 one side, as by a drop of sealing-wax, has been passed horizontally through its 

 summit, the small twisting force is sufficient, as de Vries has shown, to cause a 

 permanent torsion in the growing part. The same result will of course ensue if a 

 leaf or branch instead of a pin is attached to the side of the organ. Branches which 

 grow horizontally and bear decussate pairs of leaves usually exhibit alternate tor- 

 sions of their internodes to the right and left, so that the leaves all stand in two 

 rows along the branch instead of in four. De Vries has shown that this is occasioned 

 by the unequal twisting force of the leaves of each pair. If the young leaves 

 are cut away no torsion results ; if only one of each pair is removed, the torsion is 

 determined by the weight of the remaining leaf. 



Torsions of this kind also occur frequently when leafy shoots rise in consequence 

 of geotropism from a horizontal position, and are caused by the unequal distribution 

 of the weight of the leaves, and by their various geotropic and heliotropic curvatures 

 twisting the stem as it becomes erect. Very clear instances are furnished by long 

 petioles as those of Cucurbita, when the branch from which they spring is fixed in 

 a reverse position. The effect of geotropism alone or combined with heliotropism 

 would be simply to cause the petiole to curl upwards in a vertical plane ; but the 

 weight of the lamina is scarcely ever equally distributed on the two sides of the plane 

 of curvature ; one side is more heavily weighted, and causes the plane of curvature 

 of the petiole to bend obliquely to that side, and other parts of the petiole to be 

 .thus exposed to the influence of gravitation and heliotropism. Complicated curva- 

 tures and torsions of the petiole and of the lamina itself are caused in this way, the 

 final result being again to reverse the lamina, so as to bring its proper upper side 

 uppermost and expose it to the light as much as possible. 



It will be seen therefore that a distinction must be drawn between two kinds of 

 torsion; firstly, that of erect organs; and secondly, that of organs which grow in a hori- 

 zontal or oblique position. In the former case the torsion results from internal con- 

 ditions of growth, and especially from the outer layers growing more rapidly than the 

 inner ones ; the arrangement of the internal parts — in the internodes of higher plants 

 probably the course of the fibro-vascular bundles — determines the direction of the torsion. 



Torsions of the second kind are caused in quite a different way. The outer layers 



of the growing organ are in a state of passive tension, and there is no internal tendency 



. to torsion ; but the weight of the parts attached to it causes a torsion of the growing 



organ, which is rendered permanent by growth and by the very imperfect elasticity 



of the organ. 



