Open main menu

Page:Popular Science Monthly Volume 18.djvu/521

This page has been validated.
505
DARWIN ON THE MOVEMENTS OF PLANTS.

Rc is the right, and Lc the left one, stand directly opposite one another, and the first true leaf (F) projects at right angles to them. At night (see II and III) the right cotyledon (Rc) is greatly raised, but is not otherwise changed in position. The left cotyledon (Lc) is likewise raised, but it is also twisted so that its blade, instead of exactly facing the opposite one, now stands at nearly right angles to it. This nocturnal twisting movement is effected by the twisting of the whole length of the petiole. At the same time the true leaf (F) rises up vertically, or even inclines inward. It also twists a little, so that the upper surface of its blade fronts the upper surface of the twisted left cotyledon. The whole case is remarkable, as with the cotyledons of no other plant have we seen any nocturnal movement except vertically upward or downward."

The various ways in which the leaves of plants are protected from loss of heat by radiation at night are shown by diagrams and pictures from which we select some of the most striking. A good deal of space is given to an account of the circumnutation and nyctitropic movements of the Oxalidæ. In most of the species of oxalis the three leaflets sink vertically down at night. But, as their sub-petioles are short, the blades could not assume this position from want of space, unless they were in some manner rendered narrower, and this is effected by their becoming more or less folded (Fig. 6), so that their lower surfaces are brought near together (see B), as if the object were their protection rather than that of the upper surface. This would form a marked exception to the rule, that the object of sleep is protection of the upper surfaces from radiation, if it had not been found that, in species where the sub-petioles are longer, the leaflets sink without folding together. By thus crowding together at night, a much smaller surface is exposed than during the day.

PSM V18 D521 Medicago marina in durnal and nocturnal state.jpg
Fig. 7.—Medicago marina: A, leaves during the day; B, leaves asleep at night.

"The drawing of Medicago marina, awake and asleep (Fig. 7), answers almost as well for Cytisus fragrans, which rose at night on one occasion 23° and on another 33°. The three leaflets also bend upward, and at the same time approach each other so that the base of the central leaflet overlaps the bases of the two lateral leaflets. They