Revision Notes: Chemical Coordination in Plants | Biology Class 10 ICSE PDF Download

Introduction

Plant hormones, also known as phytohormones, play a crucial role in regulating various aspects of plant growth, including cell division, cell enlargement, and cell differentiation. These hormones are produced in specific parts of the plant and can significantly influence their development.

1. Auxins
   • Auxins are responsible for promoting the growth of plants.
   • They are secreted by the cells present in the tips of stems and roots.
   • Synthetic auxins are used in horticulture to encourage plant growth.
2. Gibberellins
   • Gibberellins promote cell differentiation, but this process occurs in the presence of auxins.
   • They are also involved in breaking seed dormancy, stimulating the elongation of shoots.
3. Cytokinins
   • Cytokinins play a vital role in promoting cell division in plants.
   • They also help delay the ageing of leaves, promote the opening of stomata, and stimulate fruit growth.
4. Abscisic Acid
   • Abscisic acid acts as a growth inhibitor and promotes dormancy in seeds and buds.
   • It also encourages the closing of stomata, wilting and falling of leaves, and the detachment of flowers and fruits from plants.

Movement Due to Growth

Movement of plant organs towards or away from a stimulus is known as tropism.
Since the tropic movements are slow, the stimulus needs to be continued for a longer time for the effects to be noticed.
Different types of tropic movements in plants:

1. Phototropism
   • Movement of plant parts towards or away from light is termed phototropism.
   • Because shoots of most plants grow towards the source of light, it is termed positive phototropism.
   • Roots grow away from light and hence are negatively phototropic.
2. Geotropism
   • Movement of plant organs in response to gravity is termed geotropism.
   • Roots are positively geotropic because they grow in the direction of gravity.
   • The shoot grows upwards, i.e. against gravity, and hence is negatively geotropic.
3. Thigmotropism
   • Movement of plant organs in response to stimuli caused by physical contact with solid objects is termed thigmotropism.
   • Weak-stemmed plants use twining stems and tendrils to climb on other plants/objects which provide them support. Hence, twining stems and tendrils are positively thigmotropic.
4. Hydrotropism
   • Movement of plant organs in response to water is termed hydrotropism.
   • Roots grow towards the source of moisture and hence are positively hydrotropic.
5. Chemotropism
   • Movement of plant organs in response to a chemical stimulus is called chemotropism.
   • When plant organs grow away from the chemical response, it is called negative chemotropism.
   • When plant parts grow towards the chemical response, it is called positive chemotropism. For example, pollen tubes grow towards the sugary substance secreted by the stigma of the flower.
6. Heliotropism
   • Diurnal motion or seasonal motion of plant parts in response to the direction of the Sun is termed heliotropism.
   • Sunflowers contain auxins which are sensitive to sunlight.
   • They stimulate the growth of the cells in the shaded region of the stem so that the flowers end up bending in the opposite direction, i.e. towards the Sun.