Question

Examples of positive geotropism
(a) Closing of flowers
(b) Upwards growth of stem
(c) Downwards growth of root
(d) Lateral growth of root

Answer 1

Geotropism or gravitropism (“gravity-turn”) is the ability to move in response to gravity.

Complete answer: Roots display positive geotropism, in their responses to gravity, and the shoots exhibit negative geotropism.
Geotropism takes place as soon as the initial germination signs of seed are visible. This ensures that the root grows into the soil and the shoot grows toward sunlight, regardless of how the seed is placed when it lands.

Additional information: In plants, the detection of gravity is by the help of statoliths which are dense cytoplasmic components that settle under the influence of gravity in the lower portions of the cell. In vascular plants, there are specialised plastids which have numerous dense starch grains.
Statoliths are found in certain cells of the root cap.When the direction of the gravitational pull changes, the root cap cells sense it and it results in a change in auxin distribution in the top of the root.

So, the correct answer is ‘Downward growth of the root’

Note: The way root cells have a response to the high concentration of auxin and redistribution during the gravitropic response is completely opposite to the way that cells in the stem respond during phototropism.
In stems, high concentrations of auxin lead to increased cell elongation and bending. In roots, high concentrations of auxin lead to decreased cell division and elongation. Roots bend as cells on the upper side of the zones of cellular division and elongation continue to grow.
Recently biologists demonstrated precisely which cells are involved in gravity sensing in Arabidopsis roots.