Why does stem of a tree always grow upward and roots grow downward into the ground?

Stem and roots (and seeds to start with) of trees have two natural properties known as phototropism and geotropism respectively. Bending and bowing of a sapling in any direction or degree according to circumstances is known as tropism. Only external stimulus is responsible for bending or stretching of sapling. The seed has no role in it. Whatever the position of seed, whether upright or sideways, the bud of stem will raise upward from the ground and the root fibers will burrow downward in the ground.

The stem of every tree has property of phototropism. Photo means pertaining to light and tropism means pertaining to bending. Simply put, stem always tries to bend in the direction of sunlight. If you put a potted plant near an open window then the stem of plant will bend towards the window on its own. Thereafter, if you turn the pot in opposite direction, the plant will again bend toward the window after some time. This type of bending is due to a plant hormone named Auxin. Secretion of hormones creates longer cells on one side of stem than the opposite side. This uneven growth of cells causes the stem to bend toward the direction in which there are small cells. The opposite side having bigger cells assists the bending process. Main advantage of this property is that the plant or vine does not have to become mobile. It can make movement towards the most suitable direction.

Charles Darwin was first to be intrigued by this phenomenon. Thinking that the tip of plant contained some substance which recognized sunlight, he covered the tip with an opaque cap. His assumption turned out to be correct. The tip of plant was no longer attracted towards sunlight. Darwin found an explanation but it was incomplete. About one hundred years later a botanist named Schwitz Vent discovered hormones responsible for the growth of plants and trees. This chemical was named Auxin by the subsequent researchers. They observed from experiments that less Auxin was secreted on shaded side of stem. In this condition the stem would invariably bend towards the direction of sunlight.

Roots of plants and trees have natural property of geotropism because gravity pulls their growth hormones to the lower part. Roots are not only partial to gravitation of earth but also have aversion to sunlight, with the results that roots automatically get embedded in ground. Perhaps the highest degree of geotropism is displayed by the Indian fig (banyan) tree. The roots of a vary odd banyan tree in erstwhile Transvaal Republic of South Africa have reached 120 meters (400 feet) deep in the Earth. From the point of view of length of root none can match rye plant. One single plant of a variety of rye is known to have spread a vast network of roots whose aggregate length was 623 kilometers.

Additional reading:
Tropism (Wikipedia)
Phototropism (Wikipedia)
Gravitropism (Wikipedia)

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