When the leaflets sink
vertically down at night and the petioles rise, as often occurs, it is
certain that the upward movement of the latter does not aid the leaflets in
placing themselves in their proper position at night, for they have to move
through a greater angular space than would otherwise have been necessary.
Notwithstanding what has just been said, it may be strongly suspected that
in some cases the rising of the petioles, when considerable, does
beneficially serve the plant by greatly reducing the surface exposed to
radiation at night. If the reader will compare the two drawings (Fig. 155,
p. 371) of Cassia pubescens, copied from photographs, he will see that the
diameter of the plant at night is about one-third of what it is by day, and
therefore the surface exposed to radiation is nearly nine times less. A
similar conclusion may be deduced from the drawings (Fig. 149, p. 358) of a
branch awake and asleep of Desmodium gyrans. So it was in a very striking
manner with young plants of Bauhinia, and with Oxalis Ortegesii.
We are led to an analogous conclusion with respect to the movements of the
secondary petioles of certain pinnate leaves.
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