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Responses to Stimuli
Plants respond to changes in their internal and external environments. Stimuli are anything that initiates a
response or action in an organism. Plants respond to external stimuli including light, heat, and moisture.
Plants respond to internal stimuli too, including water and nutrient levels within the plant cells. Responses to
external and internal stimuli helps plants meet their basic needs and maintain homeostasis.
Plants are sessile, meaning they cannot move from place to place. However, plants do have a limited range
of motion toward and away from specific stimuli. Movement in response to a stimulus is called tropism. Plant
tropisms include:

Thigmotropism occurs when a plant responds to the touch of another object.

Phototropism: Plants tend to move and grow toward the Sun in order to
maximize the amount of sunlight they receive.
 Thigmotropism: Some plants respond to touch from another object. When a
vine comes in contact with a wall or a tree, it may change its direction of growth to
wrap itself around the object as it grows.
 Gravitropism: Plants respond to the force of gravity by growing roots downward
into the soil, and by growing stems away from the soil. This helps to ensure that
roots grow downward and that shoots grow upward.
Plant hormones coordinate the tropic responses to sunlight, touch, and
gravity. Auxins are plant hormones that control plant growth and development. They
are produced in shoot tips and travel downward inside the plant. Auxins cause root formation and make new
buds grow. Auxins also inhibit the growth of lateral buds, causing plants to grow upward rather than
spreading outward.
When plants exhibit phototropism, auxin is released, causing the response. Light hitting one side of a plant
causes a higher concentration of auxin to develop on the plant's shady side. This promotes growth and
elongation of the stem on the shady side, causing the plant to bend toward the light.
Cytokinins also promote plant growth. They are hormones produced in growing roots and in developing fruits
and seeds. Similar to auxins, cytokinins stimulate cell division to promote growth. Unlike auxins, ctyokinins
cause plants to grow outward, rather than upward, by promoting growth of lateral buds.
Plants also respond to the presence of certain gases in their environment. Farmers use ethylene gas to
speed up the ripening of fruits such as bananas and tomatoes. Plants respond to the gas because they
naturally produce ethylene gas for the same purpose—to promote growth and the ripening of fruit. Some
plants release noxious gases that warn neighboring plants about the presence of a possible predator.
Cabbage and tobacco plants release the gas methyl jasmonate when pests feed on their leaves. The gas
induces pest resistance in other parts of the plant and in neighboring plants.