Whilst the two halves of the leaf are
closing, the midrib at first sinks vertically downwards and afterwards
bends backwards, so as to pass close along one side of its own upwardly
inclined petiole; the midrib being thus directed towards the stem or axis
of the plant. The angle which the midrib formed with the horizon was
measured in one case at different hours: at noon it stood horizontally;
late in the evening it depended vertically; then rose to the opposite side,
and at 10.15 P.M. stood at only 27o beneath the horizon, being directed
towards the stem. It had thus travelled through 153o.
[page 374]
Owing to this movement--to the leaves being folded--and to the petioles
rising, the whole plant is as much more compact at night than during the
day, as a fastigiate Lombardy poplar is compared with any other species of
poplar. It is remarkable that when our plants had grown a little older,
viz., to a height of 2 or 3 feet, the petioles did not rise at night, and
the midribs of the folded leaves were no longer bent back along one side of
the petiole. We have noticed in some other genera that the petioles of very
young plants rise much more at night than do those of older plants.
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