Before leaving this subject there is one set of responses to be referred to that is not the result of growth. Certain movements are brought about by the change in the turgidity of certain organs. The small lateral leaflets of _Desmodium gyrans_ make circling movements in one to three minutes. No apparent benefit results from their action. The terminal leaflets of _Trifolium pratense_ oscillate in periods of two to four hours, but do so only in the dark; in the light the leaflets assume a rigid position. There is nothing in the process to suggest that the movement is useful to the plant, and yet it appears to be as definite as are those cases in which the response is of vital importance. Had these movements been of use, their origin would, no doubt, have been explained because of their usefulness, and the conclusion would have been wrong. The leaves of the Mimosa respond, when touched, and it cannot be supposed that this is of any great advantage to the plant. The sleep movements of many plants are also due to the effect of light. In some cases the leaflets are brought together with their upper surfaces in contact with one another; in other cases the lower surfaces are brought together. Darwin supposed that these sleep movements served to protect the leaves from a too rapid loss of heat through radiation, but it has been pointed out that tropical plants exhibit the same responses. We have here another admirable instance of the danger of concluding that because we can imagine an advantage of a certain change, that the change has, therefore, been acquired because of the advantage. In the Mimosa not only do the leaflets close together, but the whole leaf drops down if the stimulus is strong. Other plants also show in a less degree the same movements, Robinia and Oxalis for instance, and certainly in these latter the result does not appear to be of any advantage to the plants.