Why shoots grow up

Plant hormones. The tips have been removed. No auxin is produced and the shoots do not grow longer. The tips have been covered so light cannot reach them. Auxin is in the same concentration on both sides of the shoots, so they grow evenly and longer on both sides.

One side of the tips are in more light than the other side. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.

If lit from above, the plant will grow upwards. Auxins have the opposite effect on root cells. In a root , the shaded side contains more auxin and grows less - causing the root to bend away from the light. Phototropism is a response to the stimulus of light, whereas gravitropism also called geotropism is a response to the stimulus of gravity. Plants responses to gravity:. In a root placed horizontally , the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.

The opposite happens in a stem. When a stem is placed horizontally , the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity. Auxin and phototropism. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.

Time-lapse of pea shoot and root growth : Time-lapse of a pea plant growing from seed, showing both the shoot and root system. The roots grown downward in the direction of gravity, which is positive gravitropism, and the shoot grows upward away from gravity, which is negative gravitropism. The reason plants know which way to grow in response to gravity is due to amyloplasts in the plants.

Amyloplasts also known as statoliths are specialized plastids that contain starch granules and settle downward in response to gravity.