krit.club logo

Control and Coordination - Tropic movements in plants

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Tropic movements are directional growth movements in plants that occur in response to an external stimulus. The direction of growth is determined by the direction of the stimulus.

•

Phototropism: Growth movement in response to light. Shoots show positive phototropism (bend towards light), while roots show negative phototropism. This is mediated by the hormone Auxin (C10H9NO2C_{10}H_9NO_2).

•

Geotropism (Gravitropism): Growth movement in response to gravity. Roots show positive geotropism (grow downwards towards gravity gg), and shoots show negative geotropism (grow upwards against gravity).

•

Hydrotropism: Growth movement of plant parts (specifically roots) towards water (H2OH_2O). Roots will bend towards a moisture source even if it means growing against gravity.

•

Chemotropism: Growth movement in response to chemical stimuli. A classic example is the growth of the pollen tube towards the ovule stimulated by sugary substances like sucrose (C12H22O11C_{12}H_{22}O_{11}).

•

Thigmotropism: Growth movement in response to touch. Climbing plants use tendrils to circle around a support; the side in contact with the support grows slower than the outer side due to auxin redistribution.

•

Mechanism of Phototropism: When unilateral light falls on a shoot, Auxin diffuses towards the shady side. The higher concentration of Auxin on the shaded side stimulates cells to elongate more than on the illuminated side, causing the shoot to bend towards the light.

📐Formulae

Stimulus Direction→Directional Growth (Tropism)\text{Stimulus Direction} \rightarrow \text{Directional Growth (Tropism)}

C10H9NO2 (Indole-3-acetic acid / Auxin)C_{10}H_9NO_2 \text{ (Indole-3-acetic acid / Auxin)}

C12H22O11 (Sucrose involved in Chemotropism)C_{12}H_{22}O_{11} \text{ (Sucrose involved in Chemotropism)}

Growth Rateshaded>Growth Ratelit  ⟹  Bending towards light\text{Growth Rate}_{shaded} > \text{Growth Rate}_{lit} \implies \text{Bending towards light}

💡Examples

Problem 1:

Explain why a plant kept near a window bends towards the sunlight using the concept of plant hormones.

Solution:

The bending is due to positive phototropism mediated by Auxin (C10H9NO2C_{10}H_9NO_2). When sunlight reaches only one side of the stem, Auxin moves to the shaded side. This causes the cells on the shaded side to grow longer than those on the bright side, resulting in the stem curving toward the window.

Explanation:

Auxin promotes cell elongation in shoots. Its unequal distribution ([Auxin]shaded>[Auxin]lit[Auxin]_{shaded} > [Auxin]_{lit}) creates a growth differential that causes the directional bend.

Problem 2:

A pollen grain lands on a stigma. Describe the movement of the pollen tube towards the embryo sac.

Solution:

This is an example of Chemotropism. The ovule secretes chemical substances (including sugars like C12H22O11C_{12}H_{22}O_{11} and calcium ions Ca2+Ca^{2+}) that act as a stimulus.

Explanation:

The pollen tube grows through the style following the chemical gradient. Since the growth is directed by a chemical concentration gradient, it is classified as a tropic movement (positive chemotropism).

Problem 3:

In an experiment, a pea plant tendril comes in contact with a wooden stake. What happens to the growth rate of the tendril at the point of contact?

Solution:

The part of the tendril in contact with the stake grows slower than the part away from the stake. This is called Thigmotropism.

Explanation:

Touch stimulus causes Auxin to move away from the point of contact. The higher concentration of Auxin on the outer side causes rapid elongation, making the tendril circle and cling to the support.