Does the Direction You Plant a Seed Affect Its Growth?

Germination

• As a seed begins the process of germination, it absorbs a large quantity of water. Enzymes are activated and cells inside the seed begin to divide. Soon the seed coat splits and a root emerges that grows downward, even if it starts at the top of the seed. Soon afterward, a shoot begins to unfold, growing upward toward the surface of the soil. Even in complete darkness, it will always grow straight up.

Gravitropism

• Gravitropism is the term for a plant's response to gravity. Roots grow toward gravity, so they are said to be positively gravitropic. Stems are said to be negatively gravitropic because they grow away from gravity. No one knows for certain how cells respond to gravity, but one hypothesis explains the process as an accumulation of auxins at the bottom of a cell. This inhibits cells at the base of horizontal root tissue, causing the roots to bend downward and elongates the cells at the base of horizontal stem tissue, causing the stems to curve upward.

Auxins

• Plant hormones, also known as growth regulators, are chemicals that are effective in very small amounts. They are often synthesized in one part of a plant and transported to other parts. Often, two or more hormones act together.

  Auxins are a group of plant hormones that control the elongation of cells, the rooting of the plant and the dominance of certain buds on the stems. Other growth regulators include gibberellins, cytokinins and abscisic acid.

Other Effects Of Gravitropism

• Though the sun is never directly overhead in northern latitudes, trees still grow straight up, indicating a response to gravity as well as to light. Even the way plants right themselves after being blown over may be an effect of gravitropism.