30 m. they
were slightly curved downwards; in 5 hrs., after being rubbed, the
end of one was bent completely back, parallel to the basal portion;
in 4 hrs. subsequently it became nearly straight again. To show how
sensitive the young petioles are, I may mention that I just touched
the under sides of two with a little water-colour, which when dry
formed an excessively thin and minute crust; but this sufficed in 24
hrs. to cause both to bend downwards. Whilst the plant is young,
each leaf consists of three divided leaflets, which barely have
distinct petioles, and these are not sensitive; but when the plant is
well grown, the petioles of the two lateral and terminal leaflets are
of considerable length, and become sensitive so as to be capable of
clasping an object in any direction.
When a petiole has clasped a twig, it undergoes some remarkable
changes, which may be observed with the other species, but in a less
strongly marked manner, and will here be described once for all. The
clasped petiole in the course of two or three days swells greatly,
and ultimately becomes nearly twice as thick as the opposite one
which has clasped nothing. When thin transverse slices of the two
are placed under the microscope their difference is conspicuous: the
side of the petiole which has been in contact with the support, is
formed of a layer of colourless cells with their longer axes directed
from the centre, and these are very much larger than the
corresponding cells in the opposite or unchanged petiole; the central
cells, also, are in some degree enlarged, and the whole is much
indurated.
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