It has, however, been clearly ascertained that this turgid state is associated with the passage of fine threads or filaments of protoplasm from one cell to another, and at the same time with an accumulation of a soluble chemical compound named glucose, a kind of sugar, in fact. This substance possesses great osmotic power; that is, it can pass very rapidly through the flexible cell-walls of the pulvinus forming the so-called springs. These movements are, therefore, closely connected with the rapid absorption and expulsion of liquid. Contrary to the habit of most plants, the sensitive plant raises its leaves at night and closes them by day. The most usual kind of movement in these plants is that in which the leaves as well as the floral envelopes assume the position they occupied before the buds opened. Compound leaves, such as the leaves of the Leguminosæ, or pea-family, exhibit a simple or compound movement. The leaves of the bean fold upward, those of the Lupinus fold downward. In Tamarinds the leaves fold to the side. In some other plants the common petiole of the compound leaves become raised or depressed, while the leaflets turn downward or sidewise. This is the case in Amorpha fruticosa and Gleditschia tracanthus. In the well-known Mimosa pudica, which is a hothouse plant in temperate regions, the leaflets fold together, the small stalks of the leaflets of the compound leaves of this plant approach each other, and the main petiole becomes depressed. In one exceedingly sensitive species of Oxalis, the pinnate leaves fold upward. A footfall is said to be sufficient to cause it to close its leaves. When these movements of leaves or leaf-organs take place at stated hours, and when the leaves remain in the new position after the movement has ceased until a particular period of time recur, the closing up is called the _sleep_ of plants. This condition is observed both in seed-leaves and true leaves, as well as in the petals of flowers. So far as can be made out, the object of this closing of the leaves seems to be to prevent the chilling effect due to radiation from being injurious to the plant. This folding up causes a smaller extent of surface to be exposed. Radiation of heat during a clear night goes on rapidly from all surfaces such as those of expanded leaves. The closing of the leaves may be supposed to form a protective covering, which prevents the heat passing away into space, and thus saves the plant from the injurious effects of cold. This is only true of the foliage leaves, which expand during the day and close during the night.