EFFECTS OF CHANGES OF LIGIIT. light comes from the sun. Without sunlight there would be no life. Were it not for the stimulus of the sun's rays, animals would he eyeless; and it is the absence of light which bleaches animals liv ing in darkness, preventing the formation and development of the pigment in the skin. On the other hand. the colors of animals. the beautiful and varied tints of butterflies and bird., of tropi cal shells as well as the gorgeous hues of flowers, are all due to the stimulating effects of light.
Light is necessary for the development of chlorophyll, and therefore is an indispensable agent in the life of all green plants. and espe cially for tree-life. and wood-formation. Sachs observes that "the forms and modes of life pre sented by plants . . . must have been to a large extent induced by the continued action of. gravitation and light." And this applies as a general statement to the simple animals, and in a sense to all animal life.
The influence of changes in the environment, and especially of light, on organism, may be im mediate and direct. or it may be indirect. The French botanist Bonnier has shown by experi ments that a life in feeble light produces a change in the structure of plants. and that if
ilaced in markedly different surroundings they either perish or become rapidly modified to meet the changed conditions. He has found that Arctic plants differ noticeably from the same species growing in Alpine regions in the greater thick ness and simpler structure of the leaves. and has attributed this to the feeble light of the Arctic region and to the greater degree of moisture. By means of feeble electric lighting and a moist, cool temperature he had in his laboratory in Paris produced these differences, i. e. converted Alpine into Arctic plants.
Pnozomxts. This means the influence of light in directing the movements of freely moving cells or unicellular plants, as certain bacteria. 'Helio tropism' means a turning to the light, as seen in the case of infuso•ia and many higher ani mals which will in an aquarium press to the light and collect on the side of the vessel next to the light. The influence of light on night flying moths and other insects is well known. lee Tumeism.