This nocturnal
twisting movement is effected not by means of the pulvinus, but by the
twisting of the whole length of the petiole, as could be seen by the curved
line of its upper concave surface. At the same time the true leaf (F) rises
up, so as to stand vertically, or it even passes the vertical and is
inclined a little inwards. It also twists a little, by which means the
upper surface of its blade fronts, and almost comes into contact with, the
upper surface of the twisted
[page 311]
left cotyledon. This seems to be the object gained by these singular
movements. Altogether 20 seedlings were examined on successive nights, and
in 19 of them it was the left cotyledon alone which became twisted, with
the true leaf always so twisted that its upper surface approached closely
and fronted that of the left cotyledon. In only one instance was the right
cotyledon twisted, with the true leaf twisted towards it; but this seedling
was in an abnormal condition, as the left cotyledon did not rise up
properly at night. This whole case is remarkable, as with the cotyledons of
no other plant have we seen any nocturnal movement except vertically
upwards or downwards.
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