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Usage examples of leaflets.

Owing to this rising of the petioles, and the vertical sinking of the large leaflets, the leaves become crowded together at night, and the whole plant then exposes a much smaller surface to radiation than during the day.

A filament was fixed to the midrib of one of the longer leaflets, and the movement of the whole leaf was traced, as shown in Fig.

The summit of the petiole was then secured to a stick, and the separate leaflets were found to be continually circumnutating.

Here we have the unusual case of one of the leaflets not standing vertically, or almost vertically, at night.

The fact that the leaves of many plants place themselves at night in widely different positions from what they hold during the day, but with the one point in common, that their upper surfaces avoid facing the zenith, often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper surfaces from being chilled at night by radiation.

This shows how much cooler the leaflets fully exposed to the zenith must have become, than those which stood almost vertically, either upwards or downwards, during the night.

Next morning ten of the terminal leaflets were examined as opaque objects under the microscope.

These leaflets, in going to sleep, either turn vertically upwards, or more commonly bend a little over the lateral leaflets, so that their lower surfaces are more exposed to the zenith than their upper surfaces.

Nevertheless, six of these ten leaflets were distinctly yellower on the upper than on the lower and more exposed surface.

Again, the tips of some few leaflets, which had been pinned close to the cork, projected a little beyond the edge, so that the air could circulate freely round them.

Conditions necessary for these movements--List of Genera and Families, which include sleeping plants--Description of the movements in the several Genera--Oxalis: leaflets folded at night--Averrhoa: rapid movements of the leaflets--Porlieria: leaflets close when plant kept very dry--Tropaeolum: leaves do not sleep unless well illuminated during day--Lupinus: various modes of sleeping--Melilotus: singular movements of terminal leaflet--Trifolium--Desmodium: rudimentary lateral leaflets, movements of, not developed on young plants, state of their pulvini--Cassia: complex movements of the leaflets--Bauhinia: leaves folded at night--Mimosa pudica: compounded movements of leaves, effect of darkness--Mimosa albida, reduced leaflets of--Schrankia: downward movement of the pinnae--Marsilea: the only cryptogam known to sleep--Concluding remarks and summary--Nyctitropism consists of modified circumnutation, regulated by the alternations of light and darkness--Shape of first true leaves.

Similar observations were made on two other leaflets, with nearly the same results.

On another occasion the petiole of a leaf was secured to a little stick close beneath the leaflets, and a filament tipped with a bead of sealingwax was affixed to the midrib of one of them, and a mark was placed close behind.

We have seen that the cotyledons, instead of sinking like the leaflets, rise up vertically at night.

The leaflets are small, of a paler green and more tender consistence than the foliaceous petioles.