Tropisms are curvature movements of plant organs induced by directional environmental stimuli. Phototropism is the curvature of a plant organ induced by an external gradient of light. It manifests itself in seedlings, shoots and reproductive organs. On the other hand, hydrotropism is the growth response of an organism to water or moisture. Gravitropism is a curvature movement of a plant organ with respect to the direction of gravitational acceleration and thigmotropism are plant growth responses to contact, allowing roots to grow around various obstacles.
Explanation:
Plants have certain movements towards certain stimuli, for example, the roots seek the earth; the stem and leaves grow in the direction of the light. These movements are made in the direction of those factors because they are necessary for the survival of the plant. The movements that plants make in favor or not of a stimulus is called tropism. The direction of the different tropisms depends on the direction of the stimulus and they are named according to the stimulus. Phototropism is the orientation of the plant towards the light. The leaves and the stem will grow in the direction of the light (positive phototropism), these responses to stimuli are regulated by substances called plant hormones or phytohormones. The subterranean organs (the roots) show negative phototropism, grow towards the ground in opposition to the stimulus or are insensible to light gradients. On the other hand, gravitropism refers to the curvature response of a plant organ with respect to the direction of gravitational acceleration. The response is also due to a differential growth between opposite parts of the organ and, therefore, it occurs in growth organs or in mature organs that regain the ability to grow, it will be positive because the root goes towards the ground in the roots and negative in the aerial organs. Gravitropism is a threshold phenomenon, that is, it requires a minimal stimulus to trigger the response. The dose of the gravitational stimulus is a function of the direction and magnitude of the acceleration and the presentation time. Hydrotropism is a form of tropism characterized by the growth or movement response of a cell or an organism to moisture or water, that is, directional growth of plant roots in relation to the availability of water. Thigmotropism is growth and movement in response to contact, it allows roots to grow around various obstacles and is responsible for the ability of climbing plant stems to roll over supporting structures. When the aerial parts of a plant tend to grow around a support, they have positive thigmotropism. The roots, on the other hand, have negative thigmotropism, since it allows them to avoid obstacles that appear on the ground, such as rocks.
