This response recalls the diurnal opening and closing of certain flowers. The flowers of the night-blooming cereus open only in the dusk of evening, and then emit their strong fragrance. Other flowers open only in the daytime, and some only in bright sunlight. It is sometimes pointed out that it is of advantage to some of these flowers to open at a certain time, since the particular insects that are best suited to fertilize them may then be abroad. This may often be the case, but we cannot but suspect that in other cases it may be a matter of little importance. In special instances it may be that the time of opening of the flowers is of importance to the species; but even if this is so, there is no need to assume that the response has been gradually acquired for this particular purpose. If it were characteristic of a new form to open at a particular time, and there were insects in search of food at this time that would be likely to fertilize the plant, then the plant would be capable of existing; but this is quite different from supposing that the plant developed this particular response, because this was the most advantageous time of day for the fertilization of its flowers. We can apply this same point of view, I believe, to many of the remarkable series of tropisms shown by plants, whose whole existence in some cases is closely connected with definite reactions to their environment. Let us examine some of these cases. When a seed germinates, the young stem is negatively geotropic, and, in consequence, as it elongates it turns upward towards the light that is necessary for its later growth. The root, on the contrary, is positively geotropic, and, in consequence, it is carried downward in the ground. Both responses are in this case of the highest importance to the seedling, for in this way its principal organs are carried into that environment to which they are especially adapted. It matters very little how the seed lies in the ground, since the stem when it emerges will grow upward and the root downward. The young stem, when it emerges from the soil, will turn toward the light if the illumination comes from one side, and this also may often be of advantage to the plant, since it turns toward the source from which it gets its energy. The leaves also turn their broad surfaces toward the light, and as a result they are able to make use of a greater amount of the energy of the sunlight. The turning is due to one side of the stem growing more slowly than the opposite side, and it is true, in general, that plants grow faster at night than in the daylight. Very bright light will in some cases actually stop all growth for a time. Thus we see that this bending of the stem toward the light and the turning of the leaves to face the light are only parts of the general relation of the whole plant toward the light.