. Experimental morphology. Protoplasm; Growth. §2] GEOTROPISM 397 h. Stems. — The central fact that the upward growth of stems is determined by gravity is established by the observa- tions that on the klinostat no definitely directed growth occurs, and that on the centrifugal machine the stem turns in the opposite sense to that of the cen- trifugal pressure. The stem is nega- tively geotropic. As with roots, so with stems, a number of questions now arise : Where is the sensitive region and where the response? At what inclination of the stem is the strongest geotropic cur- vature called forth?
. Experimental morphology. Protoplasm; Growth. §2] GEOTROPISM 397 h. Stems. — The central fact that the upward growth of stems is determined by gravity is established by the observa- tions that on the klinostat no definitely directed growth occurs, and that on the centrifugal machine the stem turns in the opposite sense to that of the cen- trifugal pressure. The stem is nega- tively geotropic. As with roots, so with stems, a number of questions now arise : Where is the sensitive region and where the response? At what inclination of the stem is the strongest geotropic cur- vature called forth? "What is the im- mediate cause of the curvature ? As Fig. 108 shows, the response of the stem of a seedling is fundamentally different from that of the radicle. In- stead of the tropism beginning at one point, and continuing there as in the root (Fig. 106), it begins close below the cotyledons of the seedling and passes downwards towards the base as far as growth is still occurring. Response, consequently, takes place along the whole "stretching" region. The sensitive re- gion also, unlike that of the root, is not confined to the tip, but extends along the entire bending stem. The position in which the strongest response is incited was believed by Sachs ('79% p. 240) to be the horizontal one, and Bateson and Daewin ('88) have confirmed this conclusion. Their method depends upon the fact that a stem placed horizontally and restrained for several hours from taking the verti-. FiG. 108. —Course of geotro- pism in a plumule. Tbe successive figures 1 to 16 indicate successive stages in tlie geotropic turning of a seedling growing in lialf darlmess. Placed at first horizontal as at 1, the plant has become completely erect at 16. The most rapid growth is just behind the cotyle- dons and diminishes toward the base. The temporary bending beyond the vertical is to be noted. (FromSxEAS- BURGER, Noll, Schenck, and ScHiMPER, Textbook of Botany, Macmillan.) cal position w
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