Plant Tropism

Plant tropism TROPISM (Greek tropē, “a turning”) -fixed, automatic, inherited movements in response to particular stimuli Types of Tropism POSITIVE TROPISM- Movement toward the source of stimulation NEGATIVE TROPISM- Movement away from the source of stimulation PLANT TROPISM In 1880, Charles Darwin pioneered in plant tropism work by showing that growing tips of plants bend towards a light source which is called phototropism. He also observed that some shade plants turned away from a bright light source which is a negative form of phototropism. The turning is due to the action of the plant hormone auxin, which causes elongation. PHOTOTROPISM- the tendency of an organism to grow toward or away from a source of light This is caused by an imbalance in the distribution of auxin. Because more auxin is present, the cells on the darker side are able to elongate more than the cells on the lighted side, causing the plant to bend toward the light.

HELIOTROPISM- phototropism stimulated by sunlight GEOTROPISM- plant growth or movement in response to gravity. When the plant is horizontal, the force of gravity causes the auxin to move to the underside of the stem. Because of the increased amount of auxin, the cells on the underside of the stem elongate more than the cells on the upper part, causing the stem to turn upward.

Upward growth of plant parts, against gravity, is called negative geotropism, and downward growth of roots is called positive geotropism.

THIGMOTROPISM (GREEK thigma, touch)- a directional growth movement tropism of a plant part, especially a tendril, in response to physical contact with a surface HYDROTROPISM- movement by a plant part such as a root toward or away from a source of water SKOTOTROPISM- the orientation of growth tips on some vines toward the darkest place on the horizon

TROPISM

STIMULUS

Positive light phototropism Positive gravity gravitropism Positive touch or thigmotropism pressure

Negative touch or thigmotropism pressure

Positive water hydrotropism availability

BIOLOGICAL CHANGE RESPONSE Cell elongation is stimulated on Growing shoot bends the shaded side of a stem but towards the light. inhibited on the lighted side. Cell elongation is inhibited on the Growing root bends lower portion of the root but downwards towards the stimulated on the upper portion. centre of the gravity. Cell elongation is inhibited on the Growing shoot coils stem side touching an object but around the object. For stimulated on the non-touching example, in some side climbing vines. Cell elongation is stimulated on the stem side touching an object Shoot grows away from but inhibited on the non-touching an obstacle. side. Root tips closest to a source of water and soluble minerals Growth of the root simply grow faster than those towards a water source. further away. (Hormones not involved here.)