frog-larva, are not due to the action of the blood-cells themselves, but to the flow of the blood caused by the beat of the heart. An important factor in the life of many organisms, especially the higher plants, is the physical phenomenon called _imbibition_; it consists in the penetration of water between the molecules of solid bodies (drawn to them by molecular attraction), and the consequent displacement of the molecules by the fluid. In this way the volume of the solid body is increased, and movements are produced which may have the appearance of vital processes. The energy of these imbibitional bodies is notoriously very powerful; we can, for instance, split large blocks of stone by the insertion of a piece of wood dipped in water. As the cellulose membrane of plant-cells has this property of imbibition in a high degree (either in the living or the dead cell), the movements it causes are of great physiological importance. This is especially the case when the imbibition of the cell wall is one-sided, and causes a bending of the cell. In consequence of the unequal strain in the drying of many fruits, they split open and project their seeds to some distance (as do the poppy, snap-dragon, etc.). The moss-capsules also empty their spores as a result of imbibition-curving (in the teeth of the openings of the spore-cases). The hygroscopic points of the heron-bill (_erodium_) curl up in the dry state and stretch out when moist; hence they are used as hygrometers in the construction of meteorological huts. The so-called "resurrection plants" (_anastatica_, the rose of Jericho, and _selaginella lepidophylla_), which close up like a fist when dry, spread their leaves out flat when moistened (the leaves imbibing strongly on the inner side). There is no more real case of "resuscitation" (as many believe) in these cases than in the mythological resurrection of the body. However, these phenomena of imbibition are not active vital processes; they are independent of the living plasm, and due solely to the physical constitution of the dead cell-membranes.