Question

Explain the reason for the following with the help of diagrams. Why do the apical parts of the stem bend toward light and root towards the earth?

Answer 1

Organ curvature developing when a favourable orientation to environmental factors is violated can be described as a "tropical response". Environmental factors only act as triggers that change metabolic pathways in such a way that the energy supply metabolic system allows reactions to occur.

Complete answer:
Phototropism -This type of movement is caused by light. Not all plants and not all parts respond equally to these stimuli. In general, most of the time the stem grows and rotates towards the light source while the roots move away from it. Leaves also respond positively to light sources. However, the leaf takes a position where the wide surface of the blade is at right angles to the light rays. Therefore, the stem must be positive phototropic, root negative phototropic and transverse leaf phototrophic or diageotropic. Phototropism is also known as heliotropism.
Reason for stem bending
In this example, the positive and negative curves shown by the stems and roots, respectively, are the result of the uneven growth of the illuminated and shaded upper sides. On the stem, the growth is larger than the shaded side, while on the roots it is more than the shaded side. The greater growth of stems on the shaded side is due to the accumulation of more auxins on the shaded side than on the shaded side.

Geotropism-Any response to the stimuli of Earth's gravity is called geotropism. The effect of gravity on plants cannot be compared to light and temperature because they are both long-lasting and constant. Primary roots and some other parts of the root system tend to grow directly towards the centre of gravity and are therefore called geotropic.
The stem usually grows far from the centre of gravity and is therefore negative geotropic. However, the stems of the prostate plant have lost their negative geotropism and have even developed into a tuberous plant that acts as a root.

Geotropism can also be explained by a different (uneven) distribution of auxins. In horizontal plants, auxin buildup on the underside of the stem and root tips is caused by gravity. In the stem, a higher auxin concentration at the bottom of the peak stimulates growth on that side. This leads to the upward growth of the stem and shows negative geotropism.

Note: Some plants, such as Arachis hypogea (peanut), show more complex changes over a short period. The flower stalks of this plant initially show positive phototropism to produce flowers. Immediately after fertilization, the stems will shrink and eventually bury the developing pods underground, exhibiting negative phototropism.
Most leaves take their position at right angles to the centre of gravity and are therefore referred to as transverse geotropic or diageotropic.