This clearly proves that during the
revolving movement the internodes become bowed in every direction.
The movement is, in fact, a continuous self-bowing of the whole
shoot, successively directed to all points of the compass; and has
been well designated by Sachs as a revolving nutation.
As this movement is rather difficult to understand, it will be well
to give an illustration. Take a sapling and bend it to the south,
and paint a black line on the convex surface; let the sapling spring
up and bend it to the east, and the black line will be seen to run
along the lateral face fronting the north; bend it to the north, the
black line will be on the concave surface; bend it to the west, the
line will again be on the lateral face; and when again bent to the
south, the line will be on the original convex surface. Now, instead
of bending the sapling, let us suppose that the cells along its
northern surface from the base to the tip were to grow much more
rapidly than on the three other sides, the whole shoot would then
necessarily be bowed to the south; and let the longitudinal growing
surface creep round the shoot, deserting by slow degrees the northern
side and encroaching on the western side, and so round by the south,
by the east, again to the north.
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