Although the nyctitropic movements of leaves and cotyledons are wonderfully diversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at night, that its upper surface is exposed as little as possible to full radiation.  We cannot doubt that this is the object gained by these movements; and it has been proved that leaves exposed to a clear sky, with their blades compelled to remain horizontal, suffered much more from the cold than others which were allowed to assume [page 561] their proper vertical position.  Some curious facts have been given under this head, showing that horizontally extended leaves suffered more at night, when the air, which is not cooled by radiation, was prevented from freely circulating beneath their lower surfaces; and so it was, when the leaves were allowed to go to sleep on branches which had been rendered motionless.  In some species the petioles rise up greatly at night, and the pinnae close together.  The whole plant is thus rendered more compact, and a much smaller surface is exposed to radiation.

That the various nyctitropic movements of leaves result from modified circumnutation has, we think, been clearly shown.  In the simplest cases a leaf describes a single large ellipse during the 24 h.; and the movement is so arranged that the blade stands vertically during the night, and reassumes its former position on the following morning.  The course pursued differs from ordinary circumnutation only in its greater amplitude, and in its greater rapidity late in the evening and early on the following morning.  Unless this movement is admitted to be one of circumnutation, such leaves do not circumnutate at all, and this would be a monstrous anomaly.  In other cases, leaves and cotyledons describe several vertical ellipses during the 24 h.; and in the evening one of them is increased greatly in amplitude until the blade stands vertically either upwards or downwards.  In this position it continues to circumnutate until the following morning, when it reassumes its former position.  These movements, when a pulvinus is present, are often complicated by the rotation of the leaf or leaflet; and such rotation on a small scale occurs during ordinary circumnutation.  The many diagrams showing the movements of sleeping and non-sleeping leaves and coty-[page 562] ledons should be compared, and it will be seen that they are essentially alike.  Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as in the case of climbing plants, and secondly by its being rendered periodic in relation to the alternations of day and night.  But there is frequently a distinct trace of periodicity in the circumnutating movements of non-sleeping leaves and cotyledons.  The fact that nyctitropic movements occur in species distributed in many families throughout the whole vascular series, is intelligible, if they result from the modification of the universally present movement of circumnutation; otherwise the fact is inexplicable.