--This subject does not possess much
interest, as it is not known that sensitiveness of this kind is of any
service to seedling plants. We have observed cases in only four genera,
though we have vainly observed the cotyledons of many others. The genus
cassia seems to be pre-eminent in this respect: thus, the cotyledons of C.
tora, when extended horizontally, were both lightly tapped with a very thin
twig for 3 m. and in the course of a few minutes they formed together an
angle of 90o, so that each had risen 45o. A single cotyledon of another
seedling was tapped in a like manner for 1 m., and it rose 27o in 9 m.; and
after eight additional minutes it had risen 10o more; the opposite
cotyledon, which was not tapped, hardly moved at all. The cotyledons in all
these cases became horizontal again in less than half an hour. The pulvinus
is the most sensitive part, for on slightly pricking three cotyledons with
a
[page 126]
pin in this part, they rose up vertically; but the blade was found also to
be sensitive, care having been taken that the pulvinus was not touched.
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