402 Evolution and Adaptation 
which the plants grow, and in this way insure the lodgement 
of the seeds. Here we have an excellent example showing 
that the negative heliotropism of the flower stalk could 
scarcely have been acquired by slight changes in the final 
direction, for only the complete change is useful to the plant. 
Intermediate steps would have no special value. 
As has been pointed out in the case of the seedling plant, 
the main stem responds positively and the roots negatively 
to gravity. In addition to this, the lateral position taken by 
the lateral roots and branches and by underground stems are 
also, in part, due to a geotropic response. In this case also 
the effect is produced by the increased growth on the upper 
side when the response is positive, and on the lower when it 
is negative. Leaves also assume a transverse position in 
response to the action of gravity, or at least they make a 
definite angle with the direction of its action. 
The most striking case of geotropic response is seen in 
plants that climb up the stems of other plants. The twining 
around the support is the result of a geotropic response of 
the sides of the stem. The young seedling plant stands at 
first erect. As its end grows it begins to curve to one side 
in an oblique position, and this is due to an increase in growth 
on one side of the apex of the shoot. As a result the stem 
bends toward the other side. Not only does the end “sweep 
round in a circle like the hands of a watch,” but it rotates on 
its long axis as it revolves. Asa result of this rotation “the 
part of the stem subjected to the action of the lateral geo- 
tropism is constantly changing ; and the revolving movement 
once begun, must continue, as no position of equilibrium can 
be attained.” This movement will carry the end around any 
support, not too thick, that the stem touches. 
Most climbers turn to the left, ze. against the hands of a 
watch, others are dextral, and a few climb either way.) 
1 These cases recall the spiral growth of the shell of the snail, but the spiral 
in the latter is due to some other factor, 
