Knight found that the stimulus which influences the root to turn downward is the force of gravity. The reaction of the root in response to this stimulus is geotropism, a turning influenced by the earth. This term is applied to the growth movements of plants influenced by the earth with regard to direction. While the motor zone lies back of the root-tip, the latter receives the stimulus and is the perceptive zone. If the root-tip is cut off, the root is no longer geotropic, and will not turn downward when placed in a horizontal position. Growth toward the earth is _progeotropism_. The lateral growth of secondary roots is _diageotropism_. [Illustration: Fig. 112. Germinating pea placed in a horizontal position.] [Illustration: Fig. 113. In 24 hours gravity has caused the root to turn downward. Figs. 112, 113.—Progeotropism of the pea root.] The stem, on the other hand, which was placed in a horizontal position has become again erect. This turning of the stem in the upward direction takes place in the dark as well as in the light, as we can see if we start the experiment at nightfall, or place the plant in the dark. This upward growth of the stem is also influenced by the earth, and therefore is a case of geotropism. The special designation in the case of upright stems is _negative geotropism_, or _apogeotropism_, or the stems are said to be _apogeotropic_. If we place a rapidly growing potted plant in a horizontal position by laying the pot on its side, the ends of the shoots will soon turn upward again when placed in a horizontal position. Young bean plants growing in a pot began within two hours to turn the ends of the shoots upward. [Illustration: Fig. 114. Pumpkin seedling showing apogeotropism. Seedling at the left placed horizontally, in 24 hours the stem has become erect.] Horizontal leaves and shoots can be shown to be subject to the same influence, and are therefore _diageotropic_. =261. Influence of light.=—Not only is light a very important factor for plants during photosynthesis, it exerts great influence on plant growth and movement. [Illustration: Fig. 115. Radish seedlings grown in the dark, long, slender, not green.] [Illustration: Fig. 116. Radish seedlings grown in the light, shorter, stouter, and green in color. Growth retarded by light.] =262. Growth in the absence of light.=—Plants grown in the dark are subject to a number of changes. The stems are often longer, more slender and weaker since they contain a larger amount of water in proportion to building material which the plant obtains from carbohydrates manufactured in the light. On many plants the leaves are very small when grown in the dark.