TORSION. 
The examples of torsion we have been considering so far are produced solely 
by internal causes; the direction in which the striae run round the axis is usually 
constant in the same species ; but other instances of torsion frequently occur which 
result 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 
direction, 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 
very elastic, the torsion will disappear when the weight is removed ; but if it is only 
very imperfectly elastic, the torsion will remain permanently, as in a twisted thread 
of 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 oi 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 diff'erent 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. 
