In physiology, such changes or actions that incites the living being or part of it to quickens its actions or any functional activity is known as stimulus.
Likewise the animals or other organisms, plants also respond to the stimulus or any changes in the environment. The only difference is that animals move from one place to another when the environmental conditions become unfavourable, whereas plants do not. So, being immotile, plants develop the potential to respond to ominous conditions by adapting the certain changes.
Plant Tropisms is the process where the plants are able to habituate themselves with the changes in their environmental conditions and grow towards or against the stimulus. These factors or stimulus are touch, light, gravity, and water.
Plant Tropisms occurs due to differential growth, where the cell division is rapid in either one part of the plants such as roots or stems than the other. This cell division leads to the growth of that particular organ which further conducts the directional growth of the entire plant. The plant hormone that promotes the growth of the plant is the ‘auxins‘. These hormones thus uphold the rise of the plant in response to stimulus and can lead it to curve or bend.
Positive Tropisms is the growth of a plant that is towards the stimulus, while Negative Tropisms is the growth of a plant that is against the stimulus. There are generally six tropic respons, seen in plants, which includes gravitropism, hydroptropism, thigmotropism, phototropism, chemotropism and thermotropism.
Phototropism:
It is also known as helitropism.The kind of movement shown by the plants in response to light. The stems only tend to turn towards the light and not the whole plant. On the contrary, the roots grow away from the source of light. Even the leaves respond positively to the light rays. So the stems are positively phototropic, while roots are negatively phototrophic and the leave is said to be diatrophic or transversely phototrophic.
Gravitropism:
The response to the stimulus of the earth’s gravity is known as geotropism. Primary roots and few other parts of the roots, grow towards the centre of gravity and is known as positively geotropic. Stems are negatively geotropism as they grow against the centre of gravity. Though prostrate plants have adrift their character of negative geotropism, instead grow like the tubers or root stock that act like roots. Geotropism is of three types: orthogeotropism, diageocropism, plageotropism.
Thermotropism:
Parts of the plants can also respond to the fluctuating temperature of their surrounding. These parts display the curvature movements in response to this kind of stimulus by taking some advantageous position. This movement is known as thermotropism.
Chemotropism:
Some of the chemicals also drive few parts of the plants to take action towards it. For example, the Pollen tubes move the ovary after the absorption of borate and calcium from style; Nepenthes close lid after getting nitrogenous food, tentacles movement in Drosera.
Hydrotropism:
The response shown by the plants in response to the stimulus of water is hydrotropism. The roots are known to exhibit a positive hydrotropic response as they bend towards the water.
Thigmotropism:
It is also known as Haptotropism. This is the movement of plants in response to contact with the solid object. Thigmotropism is seen in twiners and tendrils, which are the curvature movements.