A very great number of bacteria are unfavorably affected by light, and find proper conditions at some depth in the soil or water. It is on account of this fact that the water of frozen streams becomes more thickly inhabited by certain organisms than in the summer time, and exposure to sunlight is adopted as a hygienic measure in freeing clothing and household effects from infection. Bacteria occur abundantly in sea water at depths of two hundred to four hundred metres, and quite a number of species are to be found at eight hundred to eleven hundred metres. The distribution of fungi follows the general habit of bacteria in that they thrive best in darkness. It is to be noticed in this connection that light is also a determining factor in the distribution of the higher land plants. Thus the amount of light received in polar latitudes is quite insufficient for the needs of many species, entirely irrespective of temperature. The retarding influence of light upon growth is even more marked in the lower forms than in the higher. Such action is the result of the disintegrating effect of the blue-violet rays upon ferments and nitrogenous plastic substances. The greater massiveness of the bodies of the higher plants enables them to carry on the chemical activities in which these substances are concerned in the interior, where the intense rays may not penetrate. The attenuated and undifferentiated fungi must seek the shade, to escape the dangers of strong light, against which they have no shield. The reproductive processes are particularly sensitive to illumination. The formation of zoöspores by green felt (_Vaucheria_) may occur only in darkness, at night, or in diffuse light, and these examples might be multiplied indefinitely. Many features of the germination of spores and the growth of _protonemæ_ or _prothallia_ among the mosses, liverworts, and ferns are determined by light. Perhaps the most striking reactions of plants to light are to be seen in locomotor and orientation movements. Locomotor movements are chiefly confined to lower forms, and are most noticeable in the "swarm spores," or zoöspores of the algæ, though exhibited by spermatozoöids as well. Zoöspores may be seen collected against the side of the vessel receiving direct sunlight, while the opposite side of the vessel will be free from them. The chlorophyll bodies of green cells arrange themselves similarly. The latter bodies may move away from the exposed side of the cell if the light exceeds a certain intensity.