172). If the hypocotyl is exposed to a moderately strong
lateral light it moves quickly towards this side, travelling in a straight,
or nearly straight, line. But when the lateral light is very dim its course
is extremely tortuous, and evidently consists of modified circumnutation.
Seedlings were placed before a north-east window, protected by a linen and
muslin blind and by a towel. The sky was cloudy, and whenever the clouds
grew a little lighter an additional muslin blind was temporarily suspended.
The light from the window was
Fig. 172. Brassica oleracea: ordinary circumnutating movement of the
hypocotyl of a seedling plant.
thus so much obscured that, judging by the unassisted eye, the seedlings
appeared to receive more light from the interior of the room than from the
window; but this was not really the case, as was shown by a very faint
shadow cast by a pencil on a card. Nevertheless, this extremely small
excess of light on one side caused the hypocotyls, which in the morning had
stood upright, to bend at right angles towards the window, so that in the
evening (after 4.
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