The cotyledons of seedlings only a day old rise at night considerably,
sometimes as much as afterwards; but there was much variation in this
respect. As the pulvinus is so indistinct at first, the movement probably
does not then depend on the expansion of its cells, but on periodically
unequal growth in the petiole. By the comparison of seedlings of different
known ages, it was evident that the chief seat of growth of the petiole was
in the upper part between the pulvinus and the blade; and this agrees with
the fact (shown in the measurements above given) that the cells grow to a
greater length in the upper than in the lower part. With a seedling 11 days
old, the nocturnal rise was found to depend largely on the action of the
pulvinus, for the petiole at night was curved upwards at this point; and
during the day, whilst the petiole was horizontal, the lower surface of the
pulvinus was wrinkled with the upper surface tense. Although the cotyledons
at an advanced age do not rise at night to a higher inclination than whilst
young, yet they have to pass through a larger angle (in one instance
amounting to 63o) to gain their nocturnal position, as they are generally
deflected beneath the horizon during the day.
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