Plants are fixed at a place with their roots in the ground, so they cannot move from one place to another. That is plant do not show movement of the entire body. However, movement of the individual parts or organs of a plant like root, shoot, leaves etc, are possible when they are subjected to external stimuli like light, force of gravity, chemical substances, water etc.
These movements of the plant part are usually caused by an unequal growth in its two regions by the action of plant hormones, under the influence of the stimulus. Also, a growth movement of a plant part in response to an external stimulus in which the direction of stimulus determines the direction of response is called tropism. Thus, we can say that tropism is a directional movement of the part of a plant caused by its growth.
If the growth or movement of a plant part is towards the stimulus, it is called positive tropism and if the growth or movement of a plant is away from the stimulus, then it is called negative tropism.
There are five common stimuli in the environment: light, gravity, chemicals, water and touch or contact. These five stimuli give us five types of tropisms in which one is phototropism when the stimulus is light.
What is phototropism?
The movement of a plant part in response to light is called phototropism. Or we can say that the response of a plant to light. If the plant parts i.e. stem moves towards light, it shows positive tropism. And if plant part that is root moves away from light, then it shows negative tropism.
What is Nastic movement in plants?
Now let us see how seedlings grow in light?
If the seedlings are covered with caps at the tip then also seedlings grew straight and the seedlings with the basis covered bent towards the light. This is nothing but phototropism movement only. In fact Darwin also concluded that plants are able to "see" light using the tips of the plant shoots, rather than through the stalks.
Do you know that plants are able to grow with the help of hormones like auxins and gibberellins? Auxin hormone helps plant to grow taller. It increases the flexibility of plant cell walls which are the main factor limiting plant cell growth. It is distributed evenly in the shoot. As a consequence, plant cells tend to grow faster in the presence of auxin. Gibberellins are plant hormones that are essential for many developmental processes in plants, including seed germination, stem elongation, leaf expansion, pollen maturation and the induction of flowering etc.
When plant is placed in a shaded region and sufficient amount of light does not reach but it comes from an angle. Then, amazing is that auxin start concentrating on the shaded side of the plant and the part of the plant in the sunny region remains same but the plant in the shaded region grows longer. It is because auxin is evenly distributed all around the shoot.
Therefore, from the article we have learnt that what is tropism, how plant shows movement, what is phototropism and what are the roles of auxin and gibberellin hormone in plants.