Plant responses can be grouped into tropisms and nastic responses.A tropism is a growth response of part of a plant to a directional stimulus. The direction of growth depends on the direction of the stimulus. Growth towards the stimulus is a positive tropism; growth away from it is a negative tropism.- Phototropism (light): shoots are positively phototropic, so leaves are held in the light; roots are usually negatively phototropic.
- Geotropism, or gravitropism (gravity): roots are positively geotropic and grow down into the soil; shoots are negatively geotropic and grow upwards.
- Hydrotropism (water): roots are positively hydrotropic.
- Thigmotropism (touch): the tendrils of climbing plants such as peas coil around supports.
- Chemotropism (chemicals): pollen tubes grow towards the ovule.A nastic response is a non-directional response to a stimulus: the direction of the response does not depend on the direction of the stimulus. Examples include the leaflets of Mimosa pudica folding when touched, flowers such as tulips opening in warmth and closing in cold, and the leaves of the Venus flytrap snapping shut. Some nastic movements are fast and are caused by rapid changes in the turgor of cells rather than by growth.Plants also respond to herbivores and pathogens chemically, for example by producing tannins or alkaloids, and to drought by closing their stomata.Most tropisms are brought about by uneven growth, with cells on one side of a shoot or root elongating more than those on the other side, controlled by plant growth regulators.
- Geotropism, or gravitropism (gravity): roots are positively geotropic and grow down into the soil; shoots are negatively geotropic and grow upwards.
- Hydrotropism (water): roots are positively hydrotropic.
- Thigmotropism (touch): the tendrils of climbing plants such as peas coil around supports.
- Chemotropism (chemicals): pollen tubes grow towards the ovule.A nastic response is a non-directional response to a stimulus: the direction of the response does not depend on the direction of the stimulus. Examples include the leaflets of Mimosa pudica folding when touched, flowers such as tulips opening in warmth and closing in cold, and the leaves of the Venus flytrap snapping shut. Some nastic movements are fast and are caused by rapid changes in the turgor of cells rather than by growth.Plants also respond to herbivores and pathogens chemically, for example by producing tannins or alkaloids, and to drought by closing their stomata.Most tropisms are brought about by uneven growth, with cells on one side of a shoot or root elongating more than those on the other side, controlled by plant growth regulators.