Question

Movement of Sunflower towards the direction of Sun
(a) Heliotropism
(b) Phototropism
(c) Nyctinasty
(d) Seismonasty

Answer 1

The phenomenon named by A. P. de Candolle in which a plant’s branch moves in response to sunlight due to the presence of a plant hormone called auxin to attain more sunlight for photosynthesis.

Complete step by step answer:
Sun is an important source of light for all the organisms in the world. If a plant grows in response to light, that phenomenon is called phototropism. If a plant moves in response to sunlight during daytime or seasonally it is called heliotropism. Plants like sunflowers show heliotropism.
Additional information:
- Phototropism is the general response of a plant towards light and it is of two types namely positive phototropism and negative phototropism. If a plant grows in the direction of light it is called positive phototropism and if it grows away from the light it is called negative phototropism
- If a plant grows towards the ground it is called geotropism.
- phototropism is seen in higher plants and some algae
- If a plant grows in response to darkness that phenomenon is called nyctinasty and such types are called nyctinastic plants
- If a plant responds to touch or sense they are called seismonastic plants and the phenomenon is called seismonasty.
- Seismonasty is also called thigmonasty
- The term heliotropism means ‘sun turn’
- The stems of sunflowers exhibit heliotropic movement such that their shoot apices shift from facing east in the daytime to facing west in the evening as they track the Sun’s relative position.
- Auxins are the plant hormones that are responsible for heliotropism and phototropism.

Note:
- A. P. de Candolle also named the phenomenon - heliotropism.
- Examples of phototrophic plants: white mustard.
- Examples of heliotropic plants: sunflower, Marie gold, daisies.
- Examples of nyctinastic plants: beans, peas, clove.
- Examples of seismonastic plants: Mimosa Pudica plant.