a) Gravity (b) Water (c) Light.

Aim: To show that sunlight is necessary for photosynthesis.
Materials Needed: Potted plant, black paper, boiling water, alcohol and iodine solution.
Procedure:
(i) Potted plant is placed in a dark room for 2 days to de-starch its leaves.
(ii) One of its leaves is covered with the thin strip of black paper on both sides.
(ill) The potted plant is kept in bright sunlight for 4 or 6 hours.
(iv) The selected covered leaf is plucked and the black paper is removed.
(v) This leaf is immersed in boiling water and then in alcohol to remove chlorophyll.
(vi) The leaf is now tested with iodine solution
(vii) The covered part of the leaf does not change blue-black colour because it did not receive sunlight.
(viii) The uncovered part of the leaf which received sunlight has able to synthesise starch and so it turns blue-black in colour.
Conclusion: This proves that light is essential for photosynthesis.

(i) Take pea seeds soaked in water overnight
(ii) Wait for the pea seeds to germinate
(iii) Once the seedling has grown put it in a box with an opening for light on one side.
(iv) After sometime, it is observed that the stem has bent and grown towards the light. This proves the phototropism movement of the stem.

The root and shoot move towards the direction of the stimuli, whereas the movement of the opening and closing of the flowers are not directed towards stimuli. Such movements in plants are called as Nastic movements. Unlike tropic movement, nastic movements are independent of the stimuli direction and may or may not be growth movement.

The flower of Taraxacum officinale (common Dandelion) blooms in the morning and evening it closes (Photonasty). Ipomoea alba (Moonflower) does exactly he opposite. They open in the night and hence the name moon flower and closes in the day time. Just a casual touch is enough to make the Mimosa pudica (Touch-me-not) leaves fold up and droop (Seismonasty or thigmonasty).

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. Thus the shoots are said to have positive phototropism, while the roots have negative phototropism.

Unidirectional movement of plant part to light stimulus is called Phototropism. The plant may also respond to gravity and it is termed Geotropism or Gravitropism. The response to water is called Hydrotropism. Once the shoot touches a suitable support, it grows towards the surface it is touching. This is called Thigmotropism. Growth or movement of a plant in response to chemical stimuli is called Chemotropism.

(a) To prove that sunlight is essential for photosynthesis.
(b) To destarch the plant.
(c) By Iodine test
(d) CO₂, H₂O and Chlorophyll

(a) Photonasty phenomenon is shown by the flowers of plant A and B.
(b) A- Taraxacum officinale (common dandelian). B- Ipomea alba (moon flower).
(c) Seismonasty or thigmonasty is exhibited by the leaves of plant C.
(d) Mimosa pudica leaves behave like those of plant C.