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The same cause
probably accounts for the leaves on our seedlings raised in the dead of the
winter not sleeping. Professor Pfeffer informs us that the leaves of
another species (S. Jorullensis ?) hang vertically down at night.
[page 386]
Ipomoea caerulea and purpurea (Convolvulaceae).--The leaves on very young
plants, a foot or two in height, are depressed at night to between 68o and
80o beneath the horizon; and some hang quite vertically downwards. On the
following morning they again rise into a horizontal position. The petioles
become at night downwardly curved, either through their entire length or in
the upper part alone; and this apparently causes the depression of the
blade. It seems necessary that the leaves should be well illuminated during
the day in order to sleep, for those which stood on the back of a plant
before a north-east window did not sleep.
Nicotiana tabacum (var. Virginian) and glauca (Solaneae).--The young leaves
of both these species sleep by bending vertically upwards. Figures of two
shoots of N. glauca, awake and asleep (Fig.


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