12 TWINING PLANTS. Cuap. 1. 
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. In this case the shoot would remain always 
bowed with the painted line appearing on the several 
above specified surfaces, and with the point of the 
shoot successively directed to each point of the 
compass. In fact, we should have the exact kind of 
movement performed by the revolving shoots of twining 
plants.* 
It must not be supposed that the revolving move- 
ment is as regular as that given in the above illustra- 
tion ; in very many cases the tip describes an ellipse, 
even a very narrow ellipse. To recur once again to 
* The view that the revolving H.de Vries; and the truth of this 
movement or nutation of the stems view is proved by their excellent 
of twining plants is due to growth observations. 
is that advanced by Sachs and 
