The radicle can be distinguished from
the hypocotyl only by the presence of root-hairs and the nature of its
covering. The meaning of the word circumnutation has already been
explained. Authors speak of positive and negative heliotropism,*--that is,
the bending of an organ to or from the light; but it is much more
convenient to confine the word heliotropism to bending towards the light,
and to designate as apheliotropism bending from the light. There is another
reason for this change, for writers, as we have observed, occasionally drop
the adjectives positive and negative, and thus introduce confusion into
their discussions. Diaheliotropism may express a position more or less
transverse to the light and induced by it. In like manner positive
geotropism, or bending towards the centre of the earth, will be called by
us geotropism; apogeotropism will mean bending in opposition to gravity or
from the centre of the earth; and diageotropism, a position more or less
transverse to the radius of the earth. The words heliotropism and
geotropism properly mean the act of moving in relation to the light or the
earth; but in the same manner as gravitation, though defined as "the act of
tending to the centre," is often used to express the cause of a body
falling, so it will be found convenient occasionally to employ heliotropism
and geotropism, etc.
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