The New Student's Reference Work/Geotropism
Geotropism (jḗ-ŏt′rṓ-pĭz’m) is the sensitiveness of plants to the action of the earth's gravitation, to which they respond by changing the rate of growth or the turgor (which see) of certain regions. This produces a curvature which alters the position of attached parts with reference to the direction of gravitation. Most plant-parts assume a position, fixed by the time they are mature, which is a resultant of the effect of various external factors, especially of light and gravity. But parts still capable of growth and motor-organs (which see), when put in a new position (as when an erect stem is put horizontal), curve so as to bring the parts into the normal position. The parts may be directed toward the earth (primary roots); or away from it (erect stems); or transverse to gravitation at any angle (creeping and underground stems and lateral roots).