Question

Positive phototropism is due to elongation of cells:
(a) More on the illuminated side and less on the shaded side
(b) More on the shaded side and less on the illuminated side
(c) Less on the illuminated side and a normal on the shaded side
(d) Normal on the illuminated side and less on the shaded side

Answer 1

Hint: Plants require light and water for photosynthesis; for this, they develop a process called tropism. Phototropism is the movement of plants according to its growth depending upon the light stimulus. Phototropism is of two types positive phototropism and negative phototropism.

Complete step by step answer:
- Positive phototropism is growth towards a light source and negative phototropism is growth against the light source
- Phototropism can also be observed in other organisms other than plants like fungi.
- In the plant, stem responses to light known as a positive phototropism means the stem grows towards the light and root response to light is known as negative phototropism which means the root grows away from the light.

So, the correct answer is, ”More on the shaded side and less on the illuminated side”.

Additional information:
- Auxin is a family of plant hormones mostly made in the tips of growing stem and roots known as apical Meristem and then diffuse to other parts of the stem or root.
- Auxin helps in changing the rates of elongation in plant cells and controls the growth of stem and root.
- The shaded side of stems where the presence of auxin is comparatively high, grows longer in general. Due to the same phenomenon, stems grow more where there is ample light present and restrict them to bend towards the light as well. It grows because the auxin causes the cells to elongate on the shaded side. So this side grows more resulting in the two sides to be having unequal growth and the stem grows towards the light.
so if lit from above, the plant will grow upward.
- The effect of Auxin on the plant roots has the opposite effects as compared to the stem of the plant. Due to the excess presence of auxin in the root, the shades get darker and grow less. Thus, causing the roots to bend away from the light.

Note:
- The term auxin is applied to the IAA ( indole- 3- acetic acid). They are generally produced by the growing apices of the stem and roots from where they migrate to the region of their actions.
- The phenomenon of geotropism causes the roots to grow downwards.