Behavior In Plants
The innate behavior of plants depends mainly on growth in a given direction or movement due to changes
in water content. Plant behavior in which a plant organ grows toward or away from a stimulus is known as a tropism. A positive tropism is growth toward a stimulus, while a negative tropism is growth away from a stimulus. During positive phototropism, stems and leaves grow in the direction of a source of light. Roots exhibit positive gravitropism, growth toward gravity, while stems demonstrate negative gravitropism. Since roots grow toward water, they are said to behave with positive hydrotropism. Touch stimulates positive thigmotropism, such as vines growing on supporting surfaces.
Sometimes a part of a plant moves in a specific way regardless of the direction of the stimulus. These movements are temporary, reversible, and due to changes in the water pressure inside the plant organ. The leaves of peas and beans open in the morning and close up at night. In light, ion channels open within the pulvinus, a gland present at the leaf base. Ions and water enter the leaf, causing it to open. The reverse occurs in darkness. When pollinating insects contact cornflowers, the male stamens respond by shortening rapidly, thereby releasing pollen onto the exposed female style. In the Venus flytrap, a carnivorous plant, the touch of an insect on the leaf stimulates the triggering hairs, causing the hinged lobes of the leaf to close quickly around the unsuspecting prey.