Question

Phototropic movement of roots and stems are due to
A. Action of gravity
B. Effect of light
C. Differential hormonal effect
D. Epinasty and hyponasty

Answer 1

Hint: Like humans, the plants also secrete different hormones that affect their growth and development and also the cell function. The plants do not have glands like humans; they release hormones as the secondary metabolites that are produced during the metabolic reactions.

Complete Answer:
- Most of the plants grow continuously throughout their lives, the same as the other multicellular organisms. The plant grows through a combination of cell division and cell growth. The cell growth means the increase in the size of the plant and the cell division means the increase in the number of plant cells.
- As the plant grows in size the cell of the plant becomes specialized into different cell types because of cellular differentiation. Meristem is the key of continuous repairing and growth of the plants. It is a type of tissue that is composed of undifferentiated cells that can continue to differentiate and divide.
- The apical meristem is present in the tip or the apex of the bud and the root that allows the growth of stem and root in length and flowers and leaves to differentiate. But do you ever think why the stem moves upward and the root grows in down direction, this is because of the geotropism.
- Geotropism is the response of the plant towards gravity. Further it is divided into negative geotropism and positive geotropism. The negative geotropism means the movement of plants against the gravity also termed as phototropism and the positive geotropism means the movement of plant part towards the gravity.
- Now, phototropism in stem and root is due to the difference in the hormonal effect. Stems grow positively phototropic while the roots grow in negative geotropic. Auxin is involved in the apical dormancy and positive phototropism of stem.

Note: The increase in length is considered as the primary growth of a plant but the increase in diameter is considered as the secondary growth of the plant and it does not depend on geotropism or phototropism phenomenon. Else, it depends on the addition of xylem and phloem tissue in the vascular cambium.