Sun provides al1 forms of energies in the earth. Solar energy reaches the earth in form of electromagnetic waves. The wave length of these waves ranges between 290-5000 millimicrons. The solar spectrum is divisible into visible and invisible spectra.
(a) Visible spectrum:The saves of visible spectrum bring most of the energy on the earth. It includes the wavelength between 400-750 millimicrons. Visible spectrum is divided into seven colours. These colours are red, orange. yellow, green, blue and violet. Different wavelengths (colours) play different role in the organism. For example, red and blue wavelengths are used in photosynthesis. Plants have different pigments for absorption of different wavelengths of different colours.
(b) Invisible spectrum: It has two parts: The wavelengths below 400 are called ultraviolet rays. Most of these ultraviolet rays are filtered by the ozone layers. The wavelength above 750 millimicrons is called infra red rays. These waves pass most of heat to the earth.
Classification of plants on the basis of requirement of light
There are morphological and anatomical differences between the leaves of plants growing in sun and in shade. There are also differences between sun-exposed leaves and shade exposed leaves of the same plants. Plants as a whole adjust themselves vertically towards the light. Therefore, the plants are classified ecologically on the basis of relative requirements of sunlight or shade. There are following categories of plants:
1. Heliophytes: The plants which grow best in full sunlight are called heliophytes. Heliophytes have thicker stem, short internodes, many branches, well developed roots, thick leathery leaves with more hairs per unit area. Intense light favours the formation of flowers, fruits and seeds. Mechanical tissues and xylem bundles are well developed. Leaf tresophyll is well differentiated into palisade and spongy parenchyma. The development of palisade parenchyma takes place near the upper surface of the leaf. It protects the inner tissues from intense light.
- Sciophytes: The plants which grow best at lower light intensities are called sciophytes. Sciophytes have few branches and long stem. They have thin, large, etiolated and less hairy leaves. Cuticle of these leaves is thin. The palisade cells form few layers. Thin layered epidermis may contain chlorophyll. The spongy tissue is well developed with large intercellular spaces. Photosynthetic cells and mechanical tissue are poorly developed in these plants. They have developed many adaptations for displaying their foliage to light. They may be twiners, climbers or epiphytes.
- Facultative Sciophytes: The plants which grow best in sun but can also grow fairly well under shade are called facultative sciophytes.
- Facultative Heliophytes: The plants which although grow best at lower light intensities but can also grow well in full sunlight are called facultative heliophytes.
Ecophyiological responses of light
Light is a factor of great ecological importance It has •follow ing ecophyiological responses:
It is responsible for synthesis of food by green pigment. Only about 2% light is used in photosynthesis out of the total solar energy reaching the earth. 10% energy is used in other physiological activities. Green plants manufacture carbohydrate from water and carbon dioxide during the process of photosynthesis in presence of sunlight. The solar radiations provide energy for this process. Thus the radiant energy of the sun is converted into the chemical energy by chlorophyll. The chemical energy stored in food is utilized in various other biochemical activities in the plants.
- Opening and closing of stomata
Stomata of the leaf open in the day and close in the night. Light convert starch of the guard cells into sugars. Sugar increases their ‘osmotic pressure. It increases suction pressure in guard cells. Thus water flow into them. It increases the turgidity of the guard cells. Therefore. it causes the opening of stomata. Stomata are used for gaseous exchange and transpiration.
The response of plant to 24 hour day night cycle is called photoperiodism. Photoperiodism affects the vegetative growth -and flowering of the plants. Plants are divided into three groups on the basis of photoperiod
(a) Short day plants: The plains that –develop Bower only when the photoperiod less than a critical length are called short day plants. For example tobacco plant.
(b) Long Day Plant: The plants that develop flower only when the photoperiod. more than a critical length are called long day plants.For example Hyoscyamus.
(c) Day neutral plants: They are not affected by photoperiod. For example tomato
- Development of chlorophylls
Light is essential for development of chlorophylls. If plant is kept in darkness, its leaves become yellow and losses chlorophylls. This phenomenon is called chlorosis. The chlorophylls of some leaves are lost at different places. Such leaves are called etiolated leaves.
- Germination of seed
Sonic seeds may germinate in darkness. But light is necessary for the germination of some seeds. They are called photoblastic seeds. Light requiring seeds are placed in light before sowing. They get the light stimulus. Then they are sown and they germinate easily. For example seeds of carrot.