The rotation of the protoplasm of cells is attributed to causes similar to those which produce locomotion in the simpler plants, and these movements are strikingly like some of the movements of the protozoa in the animal kingdom. The movements of the products of cell contents having no cell-wall, such as zoospores and antherozoids, are generally caused by the rapid movement of cilia (plural of the Latin word _cilium_, an eyelid) or small filaments which cover the surface. The locomotion of certain plants, such as Diatomaceæ, is apparently not due to cilia. Sensitive plants, such as the Mimosa pudica, are strongly affected by any mechanical stimulus, and thus afford us examples of the phenomenon named “irritability.” The sleep of plants is most probably a case of irritability, and differs only in degree, not in kind. Sensitiveness in plants is affected both by light and heat. It has been experimentally proved that sensitive plants, if kept in the dark, lose their sensibility after a period of seven days, and actually die after twelve days. We know that white light is composed of light of different colors. Light is propagated in waves, and each color is distinguished by having a different wave-length from that of any other color. Red light differs, for example, from violet light in the length of its waves, and violet light differs from blue, etc. It is, therefore, not surprising to find that the different colored rays are capable of producing different effects. It has been ascertained that under the influence of green light sensitive plants die after sixteen days’ exposure, though they retain their sensibility for twelve days. When the plants were exposed to violet and blue light, their growth completely ceased. They, however, retained their vitality as well as their sensibility for three months. The effect of heat on sensitive plants has also been ascertained. The sensitiveness and periodical movements of Mimosa do not begin till the temperature of the surrounding air exceeds 15° C. The periodical movements of the lateral leaflets of the Indian telegraph plant (Desmodium gyrans) can only occur when the temperature exceeds 22° C. When the temperature of the air is 40° C., the leaves become stiff in less than an hour, and at 48° C. to 50° C. rigidity takes place within a few minutes; but when the temperature falls, the sensitiveness may again be manifested. A temperature of 52° C. not only causes loss of permanent motion, but also the death of the plant. The mechanism to which the periodic movements of plants is due is not by any means fully known. The particular circumstances which regulate the turgidity have not been, so far, determined with precision.