For the upper part of the radicle, a little
above the apex, is, as we have seen, likewise sensitive; and this
sensitiveness causes the radicle to bend like a tendril towards the
touching object, so that as it rubs over the edge of an obstacle, it will
bend downwards; and the curvature thus induced is abrupt, in which respect
it differs from that caused by the irritation of one side of the tip. This
downward bending coincides with that due to geotropism, and both will cause
the root to resume its original course.
As radicles perceive an excess of moisture in the air on one side and bend
towards this side, we may infer that they will act in the same manner with
respect to moisture in the earth. The sensitiveness to moisture
* 'Arbeiten Bot. Inst., W?rzburg,' Heft iii. p. 456.
[page 199]
resides in the tip, which determines the bending of the upper part. This
capacity perhaps partly accounts for the extent to which drain-pipes often
become choked with roots.
Considering the several facts given in this chapter, we see that the course
followed by a root through the soil is governed by extraordinarily complex
and diversified agencies,--by geotropism acting in a different manner on
the primary, secondary, and tertiary radicles,--by sensitiveness to
contact, different in kind in the apex and in the part immediately above
the apex, and apparently by sensitiveness to the varying dampness of
different parts of the soil.
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