The New Student's Reference Work/Heliotropism
Heliot′ropism (in plants) is the sensitiveness
of a plant to a change in the direction of
A young plant of mustard
grown in water, showing
heliotropic curvature
caused by one-sided
illumination. n, n, level
of water; b, older part of
stem; a, younger part
where curvature occurs;
c, c, primary leaves; d,
sharp curve of root; e,
the growing point. The
arrows show the direction
of the light.
light, to which it
responds by placing its
organs again in a definite
position with
respect to it. In many
plants this is
accomplished by a change in
the rate of growth of
some part, producing
curvature and carrying
attached parts
into the new position.
Thus, if a geranium
plant be taken from a
greenhouse and set
before a window, the
leaves are carried into
a position facing the
window by curvature
of their stalks. This
is the usual response
of leaves of this kind,
while radial leaves,
as those of the onion,
direct their tips
toward the light.
Some stems bend so
as to direct the apex
toward the light, and
some roots turn away from it. Other
plants accomplish like changes by means
of the motor organs (which see), in
which changes of turgor (which see)
produce the curvatures that move the attached
parts.