There is a fourth sub-class, as far as the
final cause of the movement is concerned; for the leaves of some plants
when exposed to an intense and injurious amount of light direct themselves,
by rising or sinking or twisting, so as to be less intensely illuminated.
Such movements have sometimes been called diurnal sleep. If thought
advisable, they might be called paraheliotropic, and this term would
correspond with our other terms.
It will be shown in the present chapter that all the movements included in
these four sub-classes, consist of modified circumnutation. We do not
pretend to say that if a part of a plant, whilst still growing, did not
circumnutate--though such a supposition is most improbable--it could not
bend towards the light; but, as a matter of fact, heliotropism seems always
to consist of modified circumnutation. Any kind of movement in relation to
light will obviously be much facilitated by each part circumnutating or
bending successively in all directions, so that an already existing
movement has only to be increased in some one direction, and to be lessened
or stopped in the other directions, in order that it should become
heliotropic, apheliotropic, etc.
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