Gravitropism

• Gravitropism describes how plants grow in direct response to the force of gravity. Positive gravitropism occurs when plants grow downward; negative gravitropism happens when plants grow upward, against the force of gravity. All parts of the plant -- the stem, leaves and roots -- respond to gravity.
  
  

Roots

• When a seed germinates, the primitive root emerges first from the seed coating and anchors the plant in the soil so that above ground growth can begin. Roots respond to positive gravitropism, so they move downward in the direction of gravity. If you tip a seedling on its side, the root reorients itself and begins growing downward again in as few as 20 minutes. Early theorists thought that roots grew toward more abundant moisture found in lower levels of the soil or that the weight of the root pulled it downward, but studies that kept only the soil's surface moist or subjected seedlings to rotational forces showed that the roots were responding to gravity.
  
  

Leaves and Stems

• Leaves and stems exhibit negative gravitropism by pulling upward and against the forces of gravity. Unlike roots, these green structures require access to sunlight to produce energy for the plant. Before stems and leaves reach the surface, they depend on the limited food stores inside of their seeds. Once these stores are depleted, if the seedling has not reached a source of light, the seedling starves and dies. Gravitropism allows seedlings to quickly determine which direction is up and push toward the sunlight. If you turn a seedling or a mature plant on its side, the stems curve and the leaves tilt so that they again face upward.
  
  

Physiology of Gravitropism

• Gravitropism depends on a plant hormone called auxin. The sensitivity of a plant part to auxin depends on whether that part grows with or against gravity. For example, high levels of auxin inhibit the growth of root tips but encourage the growth of stem tips.
  
  In roots, small grains inside the cell called statoliths settle due to gravity in the bottom of the cells. These statoliths encourage the production of auxin, which affects how plants grow in response to gravity.