to same hour on following morning. Apex of leaflet 5 ½ inches from the vertical glass. Main petiole 3 3/4 inches long. Temp. 16o - 17 1/2o C. Figure reduced to one-half of the original scale.
Bauhinia (Tribe 15).--The nyctitropic movements of four species were alike, and were highly peculiar. A plant raised from seed sent us from South Brazil by Fritz Müller, was more especially observed. The leaves are large and deeply notched at their ends. At night the two halves rise up and close completely together, like the opposite leaflets of many Leguminosae. With very young plants the petioles rise considerably at the same time; one, which was inclined at noon 45o above the horizon, at night stood at 75o; it thus rose 30o; another rose 34o. Whilst the two halves of the leaf are closing, the midrib at first sinks vertically downwards and afterwards bends backwards, so as to pass close along one side of its own upwardly inclined petiole; the midrib being thus directed towards the stem or axis of the plant. The angle which the midrib formed with the horizon was measured in one case at different hours: at noon it stood horizontally; late in the evening it depended vertically; then rose to the opposite side, and at 10.15 P.M. stood at only 27o beneath the horizon, being directed towards the stem. It had thus travelled through 153o. [page 374] Owing to this movement--to the leaves being folded--and to the petioles rising, the whole plant is as much more compact at night than during the day, as a fastigiate Lombardy poplar is compared with any other species of poplar. It is remarkable that when our plants had grown a little older, viz., to a height of 2 or 3 feet, the petioles did not rise at night, and the midribs of the folded leaves were no longer bent back along one side of the petiole. We have noticed in some other genera that the petioles of very young plants rise much more at night than do those of older plants.
Tamarindus Indica (Tribe 16).--The leaflets approach or meet each other at night, and are all directed towards the apex of the leaf. They thus become imbricated with their midribs parallel to the petiole. The movement is closely similar to that of Haematoxylon (see Fig. 153), but more striking from the greater number of the leaflets.
Adenanthera, Prosopis, and Neptunia (Tribe 20).--With Adenanthera pavonia the leaflets turn edgeways and sink at night. In Prosopis they turn upwards. With Neptunia oleracea the leaflets on the opposite sides of the same pinna come into contact at night and are directed forwards. The pinnae themselves move downwards, and at the same time backwards or towards the stem of the plant. The main petiole rises.
Mimosa pudica (Tribe 20).--This plant has been the subject of innumerable observations; but there are some points in relation to our subject which have not been sufficiently attended to. At night, as is well known, the opposite leaflets come into contact and point towards the apex of the leaf; they thus become neatly imbricated with their upper surfaces protected. The four pinnae also approach each other closely, and the whole leaf is thus rendered very compact. The main petiole sinks downwards during the day till late in the evening, and rises until very early in the morning. The stem is continually circumnutating at a rapid rate, though not to a wide extent. Some very young plants, kept in darkness, were observed during two days, and although subjected to a rather low temperature of 57o - 59o F., the stem of one described four small ellipses in the course of 12 h. We shall immediately see that the main petiole is likewise continually circumnutating, as is each separate pinna and each separate leaflet. Therefore, if the movement of the apex of any one leaflet were to be traced, the course described would be compounded of the movements of four separate parts. [page 375] A filament had been fixed on the previous evening, longitudinally to the main petiole of a nearly full-grown, highly-sensitive leaf (four inches in length), the stem having been secured to a stick at its base; and a tracing was made on a vertical glass in the hot-house under a high temperature.