<P> Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the forces of gravity . In both roots and stems, auxin accumulates towards the gravity vector on the lower side . In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism). In stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion and results in the shoot curving up (negative gravitropism). </P> <P> A recent study showed that for gravitropism to occur in shoots, only a fraction of an inclination, instead of a strong gravitational force, is necessary . This finding sets aside gravity sensing mechanisms that would rely on detecting the pressure of the weight of statoliths . </P> <P> Plants possess the ability to sense gravity in several ways, one of which is through statoliths . Statoliths are dense amyloplasts, organelles that synthesize and store starch involved in the perception of gravity by the plant (gravitropism), that collect in specialized cells called statocytes . Statocytes are located in the starch parenchyma cells near vascular tissues in the shoots and in the columella in the caps of the roots . These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector . The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels . The gravitropic signal then leads to the reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in the root cap and root as a whole . The changed relations in concentration of auxin leads to differential growth of the root tissues . Taken together, the root is then turning to follow the gravity stimuli . Statoliths are also found in the endodermic layer of the hypocotyl, stem, and inflorescence stock . The redistribution of auxin causes increased growth on the lower side of the shoot so that it orients in a direction opposite that of the gravity stimuli . </P> <P> Phytochromes are red and far - red photoreceptors that help induce changes in certain aspects of plant development . Apart being itself the tropic factor (phototropism), light may also suppress the gravitropic reaction . In seedlings, red and far - red light both inhibit negative gravitropism in seedling hypocotyls (the shoot area below the cotyledons) causing growth in random directions . However, the hypocotyls readily orient towards blue light . This process is may be caused by phytochrome disrupting the formation of starch - filled endodermal amyloplasts and stimulating their conversion to other plastid types, such as chloroplasts or etiolaplasts . </P>

Study of effect of light and gravity on plant growth movement