Types of Tropisms in Plants
Tropism is a growth movement, the direction of which is determined by the direction of the stimuli. Unidirectional movement of plant part to light stimulus is called Phototropism. Heliotropism or motion of plant parts like owers or leaves in response to the direction of the sun is a kind of phototropism. The plant may also respond to gravity and it is termed Geotropism or Gravitropism. The response to water is called Hydrotropism. Climbing vines have to nd a suitable support shortly after germination. Once the shoot touches a suitable support, it grows towards the surface it is touching. is is called igmotropism. Growth or movement of a plant in response to chemical stimuli is called Chemotropism. During fertilization, pollen tube grows down the style in response to the sugars in the style is an example of chemotropism.
Tropism is generally termed “positive” if growth is towards the signal and “negative”, if it is away from the signal. While the shoot in a plant moves towards the light, the roots move away in the opposite direction. us the shoots are said to have positive phototropism, while the roots have negative phototropism. Can you think of an example of a negative hydrotropism?
When a seed germinates the primary shoot, (plumule) goes up while the primary root, (radicle) goes down. What stimulus is making the primary root to go down? Is it gravity or water that is principally responsible for the direction of the growth of a primary root? We can do an interesting experiment to find out for ourselves.
What do you need? Glass trough, sand, flower pot, plugged at the bottom, pea or bean seeds and water.
1. A glass trough is taken and is filled with sand. A flower pot containing water, plugged at the bottom is kept at the centre of the glass trough.
2. Soaked pea or bean seeds are placed around the pot in the sand, what do we observe after 6 or 7 days?
What do you observe? It will be observed that radicle has grown towards the pot and moisture instead of growing vertically downward.
What do you conclude? It proves that primary root is positively hydrotropic and that hydrotropism is stronger than geotropism in this case.