They were also able, as well as the hypocotyls of
Brassica, to form straight furrows in damp sand, whilst circumnutating and
bending towards a lateral light. In a future chapter it will be shown that
the rocking or circumnutating movement of the flower-heads of Trifolium
subterraneum aids them in burying themselves. It is therefore probable that
the circumnutation of the tip of the radicle aids it slightly in
penetrating the ground; and it may be observed in several of the previously
given diagrams, that the movement is more strongly pronounced in radicles
when they first
[page 72]
protrude from the seed than at a rather later period; but whether this is
an accidental or an adaptive coincidence we do not pretend to decide.
Nevertheless, when young radicles of Phaseolus multiflorus were fixed
vertically close over damp sand, in the expectation that as soon as they
reached it they would form circular furrows, this did not occur,--a fact
which may be accounted for, as we believe, by the furrow being filled up as
soon as formed by the rapid increase of thickness in the apex of the
radicle.
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