We now come to the nyctitropic or sleep movements of leaves. It should be remembered that we confine this term to leaves which place their blades at night either in a vertical position or not more than 30° from the vertical,—that is, at least 60° above or beneath the horizon. In some few cases this is effected by the rotation of the blade, the petiole not being either raised or lowered to any considerable extent. The limit of 30° from the vertical is obviously an arbitrary one, and has been selected for reasons previously assigned, namely, that when the blade approaches the perpendicular as nearly as this, only half as much of the surface is exposed at night to the zenith and to free radiation as when the blade is horizontal. Nevertheless, in a few instances, leaves which seem to be prevented by their structure from moving to so great an extent as 60° above or beneath the horizon, have been included amongst sleeping plants. It should be premised that the nyctitropic movements of leaves are easily affected by the conditions to which the plants have been subjected. If the ground is kept too dry, the movements are much delayed or fail: according to Dassen,[1] even if the air is very dry the leaves of Impatiens and Malva are rendered motionless. Carl Kraus has also lately insisted[2] on the great influence which the quantity of water absorbed has on the periodic movements of leaves; and he believes that this cause chiefly determines the variable amount of sinking of the leaves of Polygonum convolvulus at night; and if so, their movements are not in our sense strictly nyctitropic. Plants in order to sleep must have been exposed to a proper temperature: Erythrina crista-galli, out of doors and nailed against a wall, seemed in fairly good health, but the leaflets did not sleep, whilst those on another plant kept in a warm greenhouse were all vertically dependent at night. In a kitchen-garden the leaflets of Phaseolus vulgaris did not sleep during the early part of the summer. Ch. Royer says,[3] referring I suppose to the native plants in France, that they do not sleep when the temperature is below 5° C. or 41° F. In the case of several sleeping plants, viz., species of Tropaeolum, Lupinus, Ipomœa, Abutilon, Siegesbeckia, and probably other genera, it is indispensable that the leaves should be well illuminated during the day in order that they may assume at night a vertical position; and it was probably owing to this cause that seedlings of Chenopodium album and Siegesbeckia orientalis, raised by us during the middle of the winter, though kept at a proper temperature, did not sleep. Lastly, violent agitation by a strong wind, during a few minutes, of the leaves of Maranta arundinacea (which previously had not been disturbed in the hot-house), prevented their sleeping during the two next nights.