Tropisms and Nastic Movements
Unlike animals, plants cannot move to more favorable locations. Instead, plants survive by adjusting their growth to their local environment. A major way this is done is by sensing the directions of environmental signals such as light and gravity. This sensory information is then used to orient the direction of growth toward or away from a stimulus in a process called a tropism. By these mechanisms, shoots grow up from the ground and into thelight. This enhances photosynthesis and biomass by increasing the amount of sunlight absorbed by chlorophyll. The raised stature of the plant also promotes pollination and seed dispersal, and increases plant competitiveness.
Tropisms are different from nastic movements. Like tropisms, these plant movements are influenced by environmental cues. But the direction of a nastic movement is independent of where the signal comes from, and most such movements are temporary. Nastic movements are more specialized in function and distribution than tropisms. For example, some insectivorous plants capture prey by moving trap organs together.
Tropisms should not be confused with tactic movements found in many microorganisms, such as a unicellular green alga that moves toward the light (phototaxis). Because plants are not motile, only part of their body grows in the direction of a stimulus.
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