Movements in plants also depend upon the contractile quality of the protoplasm in the cells, and on the passage of the protoplasm from cell to cell. The property of the protoplasm gives rise to movements caused by the plant itself, which are not at least directly due to any external exciting cause. These movements can be compared with the movements of the lower animals, and to the ciliary motion found in certain tissues belonging to the most highly organized animals. The periodic movements, such as the “waking” and “sleeping” condition of leaves, the closing of flowers, etc., are manifested only when the organs are fully matured, and when the peculiarity of their internal structure which gives rise to the phenomena of periodic movements is fully developed. These movements are to be carefully distinguished from those due to unequal growth, such as movements of nutation. In this case there is no special structure upon which the movements depend. The bursting of seed-vessels, anthers, etc., is due partly to the fact that the condition of the tissues, as regards the amount of liquid they contain from their possessing unequal power of imbibing moisture, is not equally elastic. For this reason, when the less elastic portions of tissue are subjected to strain they are torn apart or bent in various ways, owing to unequal contractions and expansions, caused by an access or withdrawal of moisture. These cases can scarcely be regarded as vital phenomena, but should rather come under the category of what is in ordinary language named “warping.” They are simply caused by particular modes of the destruction of dead tissue due to conditions brought about by variations in the structure of the tissues in question. Movements in plants which take place periodically, such as sleeping and waking, or those movements that take place when they are touched or otherwise affected by certain kinds of exciting stimulus, can not be attributed to mechanical causes. The slightest mechanical stimulus on the sensitive plant Mimosa pudica causes the leaflets to fold together. Such movements are not proportional to the external stimulus, but depend on the internal structure of the plant. To this class of movements have been added the very remarkable movements which give rise to the twining condition of certain stems. Another class of movements may be mentioned, viz., movements of the protoplasm in cells, or movements of free bodies, such as zoospores (Greek, _zoon_, animal, and _spora_, seed), antherozoids (Greek, _anthos_, flower; _zoon_, animal; _eidos_, form), and sometimes even perfect individuals, such as Desmediæ, etc., which may have the power of temporary or permanent locomotion.