Principle of Phototropic and Geotropic movement in plant.

Phototropic movement: Shoots are positively geotropic. Roots are either negatively geotropic or do not respond at all .In roots ,growth is slower on the illumina4ed side relatively to the dark side, with the result that the shoot bends over towards the light.
Quantitative experiments in which shoots are subjected to light and varying intensity and duration suggests that the response is generally proportional to the total amount of light energy received by the shoot. The strong light of short duration generally produces the same degree of curvature as weak light of long duration. The resulting phototropic response is as a result of the fact that light causes an unequal distribution of hormones in the shoot, more on the dark side than the light side. This is as a result of the fact “provided there is no obstruction, auxin is transported laterally to the darker side.
Geotropic movement: This is the movement of plants in response to gravity. Because of gravity, the auxin tends to accumulate on the lower side of the shoot and root. In the shoot, this causes more growth to take place on the lower side than the upper, resulting in a upward curvature_ In the root, the reverse is the case; growth is inhibited on the lower side, resulting in a downward curvature. There are certain cells in roots and shoots, which contain special starch grains called starch statoliths. When a seedling is on its side, the statoliths fall under gravity to the lower side of the cells. It is believed that this initiates the geotropic response.
Evidence for this comes from the observation that if a vertically growing seedling is placed in a horizontal position and then returned to the vertical, bending response will be given only if the seedling was horizontal for a certain minimum period of time called the presentation time. It has been found that if you raise the temperature, the time taken for the statoliths to fall and the presentation time both decreases. Also if you destroy the statoliths (by keeping the seedling at 35°C for a couple of days) the root will no longer respond to gravity.
Although roots are generally positively geotropic and stems negatively so, there are some interesting exceptions to this. For example, the “breaking” roots of mangroves grows vertically upwards and projects from the surface of the water where they can absorb oxygen.

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