Organisms respond automatically to the luminous stimulus with an orientation and with determined movements. Whether they are provided with eyes or not, they react the same, and this reaction is not the result of sight, but a response of the living material, of the protoplasm which constitutes them, to the energy of the luminous rays. But sources of energy abound in nature and, aside from light, as Davenport has shown in his "Experimental Morphology," each one exercises a physicochemical stimulation on the living matter,-a stimulus which, acting on symmetrical organisms in the way light does, can produce, as it does, an orientation and determined automatic movements. It is to an examination of these tropisms, at least the most essential, that the first part of the present chapter will be devoted.