Botany; Geotropism; Plants -- Irritability and movements
By 9.25 A.M. on this second day the same cotyledon had
begun to fall, and a dot was made on a fresh glass. The movement was
traced until 5.30 P.M. as shown in (Fig. 8), which is given, because
the course followed was much more irregular than on the two previous
occasions. During these 8 hours the bead changed its course greatly 10
times. The upward movement of the cotyledon during the afternoon and
early part of the night is here plainly shown.
Fig. 8. Brassica oleracea: conjoint circumnutation of the hypocotyl and
cotyledons during 8 hours. Figure here reduced to one-third of the
original scale, as traced on a vertical glass.
As the filaments were fixed in the three last cases to one of the
cotyledons, and as the hypocotyl was left free, the tracings show the
movement of both organs conjoined; and we now wished to ascertain
whether both circumnutated. Filaments were therefore fixed horizontally
to two hypocotyls close beneath the petioles of their cotyledons. These
seedlings had stood for two days in the same position before a
north-east window. In the morning, up to about 11 A.M., they moved in
zigzag lines towards the light; and at night they again became almost
upright through apogeotropism. After about 11 A.M. they moved a little
back from the light, often crossing and recrossing their former path in
zigzag lines. the sky on this day varied much in brightness, and these
observations merely proved that the hypocotyls were continually moving
in a manner resembling circumnutation. On a previous day which was
uniformly cloudy, a hypocotyl was firmly secured to a little stick, and
a filament was fixed to the larger of the two cotyledons, and its
movement was traced on a vertical glass. It fell greatly from 8.52
A.M., when the first dot was made, till 10.55 A.M.; it then rose
greatly until 12.17 P.M. Afterwards it fell a little and made a loop,
but by 2.22 P.M. it had risen a little and continued rising till 9.23
P.M., when it made another loop, and at 10.30 P.M. was again rising.
These observations show that the cotyledons move
vertically up and down all day long, and as there was some slight
lateral movement, they circumnutated.
Fig. 9. Brassica oleracea: circumnutation of hypocotyl, in darkness,
traced on a horizontal glass, by means of a filament with a bead fixed
across its summit, between 9.15 A.M. and 8.30 A.M. on the following
morning. Figure here reduced to one-half of original scale.
Public-domain text, read in full here on John Shaqi.
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