But
the hypocotyl itself in some cases contracts greatly, and although at first
smooth becomes covered with zigzag ridges, as we observed with Githago
segetum. How much of the drawing down and burying of the hypocotyl of
Opuntia basilaris was due to the contraction of this part and how much to
that of the radicle, we did not observe.
Circumnutation of Cotyledons.--With all the dicotyledonous seedlings
described in the last chapter, the cotyledons were in constant movement,
chiefly in a vertical plane, and commonly once up and once down in the
course of the 24 hours. But there were many exceptions to such simplicity
of movement; thus the cotyledons of Ipomoea caerulea moved 13 times either
upwards or downwards in the course of 16 h.. 18 m. Those of Oxalis rosea
moved in the same manner 7 times in the course of 24 h.; and those of
Cassia tora described 5 irregular ellipses in 9 h. The cotyledons of some
individuals of Mimosa pudica and of Lotus Jacobaeus moved only once up and
down in 24 h., whilst those of others performed within the same period an
additional small oscillation.
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