the orientation of growing plant organs toward or away from a stimulus (gravity, light). The opposite of orthotropism is plagiotropism, that is, the orientation of plant organs toward some angle away from the stimulus. Orthotropic organs, such as the main stem or root, usually are radically symmetrical. As a plant develops, the direction of growth of its organs often changes. Lowering of temperature, changes in light conditions, or certain doses of growth stimulants may cause the shoots of some plants to shift from orthotropism to plagiotropism, resulting in the formation of creeping or prostrate forms.