. Biology; the story of living things. 374 THE MAINTENANCE OF THE INDIVIDUAL of the boulevard. The Carolina ])oplar has lost its vogue as a tree for city planting largely because of this habit of clogging drain })ipes by the response of the roots to water. The movements seen in the wilting of leaves, or the changes in the position of leaves in bright sunlight and in slight illumination, are familiar to all. There may even be a quite rapid opening and closing of flower petals, and there are also definite noticeable changes in the posi- tion of the leaflets of clover, alfalfa, oxalis, and other
. Biology; the story of living things. 374 THE MAINTENANCE OF THE INDIVIDUAL of the boulevard. The Carolina ])oplar has lost its vogue as a tree for city planting largely because of this habit of clogging drain })ipes by the response of the roots to water. The movements seen in the wilting of leaves, or the changes in the position of leaves in bright sunlight and in slight illumination, are familiar to all. There may even be a quite rapid opening and closing of flower petals, and there are also definite noticeable changes in the posi- tion of the leaflets of clover, alfalfa, oxalis, and other plants in the morning and at night. The relatively rapid responses of the leaves of the sensitive plant, Mimosa pudica, are all brought about by the functioning of struc- tures called pulvini, cushion- like enlargements of the petiole of the leaf at the point of its insertion in the stem. When the leaflets of the large compound leaves of the mi- mosa are stimulated by heat, pressure, or anesthetics, they tend to droop, the stimulus from the leaflets being trans- mitted at the extraordinarily rapid rate (for plants) of from one to three centimeters per second. When the stimulus reaches the pulvinus where the cells are large and are rich in water, a change in turgor takes place in these cells, with the result that the leaf stalk droops. In some plants there is a rapid and temporary fluctuation in growth on opposite sides of the leaves. This causes a comparatively rapid turning move- ment, but it is evident that these forces are not in themselves sufficient to explain all the changes that take place in such plants as the Leaf motility in the sensitive plant (Mimosa pudica): above, an open leaf; l)elow, a leaf whose leaflets (/) have been closed by niechanieal impact; note also that the petiole (p) has dropped; s, stipule; m, pulvinus. Mechanisms of Response in Animals The mechanism of the reflex arc has already been described in some detail in the discussion of the variou
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