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Plant Growth and Movements

Introduction

     The term growth is applied to several things and situations. It is quite common to hear people referring to growth of cities, the weeds, of tradition or even of indiscipline. You would have observed the growth of crystals or salt in the laboratory, but it is of non-living things.

Characteristics of Growth in Plants

     Growth in plants occurs by cell division and cell enlargement, followed by cell differentiation.

Growth Curve

     The curve can be shown appearing slowly along the line and stabilizing. During the initial stage, i.e., during the lag phase, the rate of plant growth is slow. Rate of growth then increases rapidly during the exponential phase.

Factors affecting Plant Growth

     Plant growth is influenced by a number of external and internal factors.

External Factors Affecting Growth

     The effect of light on growth can be studied under 3 headings light intensity, light quality and duration of light. Growth is generally favoured by darkness, but light is indispensable because of its role in the manufacture of food. Young plants growing in the absence of light develop elongated thin stems with narrow leaves and poorly developed shoot system. Such plants are known as etiolated.

Internal Factors Affecting Growth

     There are several known growth regulators. Some of them like auxins, cytokinins are growth promoting while others like abscissic acid, ethylene are growth inhibitors. Most of the growth regulators are synthesized by plants while a few are synthetic in nature.

Plant growth promoters

     The growth of plants is regulated by certain chemical substances, which are synthesized by the plant in very small quantities. These substances are formed in one tissue or organ of the plant and are then transported to other sites where they produce specific effects on growth and development.

Gibberellins

     In Japan, the farmers noted that certain rice seedlings grow taller, thinner and paler than the normal ones and toppled over. This condition was termed as bakanae disease (bakanae in Japanese means foolish) or foolish seedling disease.

Cytokinins

     Cytokinins are the phytohormones which stimulate cell division. They prevent the onset of senescence in tissues.

Ethylene

     It is commonly known that a ripe or injured fruit in a basket hastens the ripening of other fruits. Merchants generally use kerosene lamps and hay to bring about colour developments in plants quickly. It is only recently has it been realized that these effects are due to ethylene.

Abscissic Acid (ABA)

     Addicotetal (1963) found out that the shedding of cotton balls was due to a chemical substance Abscissin. Wareing and Cornforth isolated a substance that can induce bud dormancy. They named the substance as dormin. Later it was found that the 2 chemicals were identical and were given the common name Abscissic acid.

Synthetic Plant Hormones

     These are synthetic growth inhibitors which have a pronounced effect on the growth and development of plants.

Seed Germination

     The seed is the outcome of sexual reproduction in flowering plants. It represents the beginning of a new generation. A seed is defined as a fertilised mature ovule that possesses an embryonic plant, stored food material and a protective coat.

Factors affecting Seed Germination

     A dormant seed contains 10-15% of water and is generally dehydrated. So the dormant seed has to absorb water to become active and exhibit germination. Water makes the seed coat soft, causes it to rupture after swelling and start germination.

Process of Seed Germination

Seed germination is an irreversible process. Germination includes the changes that take place from the time the dry seed is provided with suitable conditions to when the seedling becomes established as an independent plant. Various changes take place during germination.

They are as follows:

Mobilisation of Reserves during Seed Germination

     During germination the cells of the embryo resume metabolic activity. The stored fats, proteins and starch are digested. The insoluble food is made soluble and complex food is made simple. Assimilation of this food by the growing embryo induces growth and the seedling assumes its shape.

Special Type of Germination: Vivipary

     This is a special type of germination found in mangrove plants. Mangrove plants are found in marshy areas and on the sea coast. In viviparous germination, the seeds germinate while still attached to the parent plant.

Dormancy

     The growth of a seed is completely arrested after it is fully developed. Such seeds will germinate if they are supplied with water and suitable temperature. In many cases the seeds do not germinate even if they are provided with all the best conditions. The completely dry ripe seed is physiologically inactive and is said to be in a resting stage. The seed is said to be dormant and the phenomenon is termed as dormancy.

Methods of Breaking Seed Dormancy

     Microorganisms present in the soil weaken and decompose the hard seed coat. The digestive juices present in the alimentary canal of the fruit eating birds makes the seed coat soft.

Role of Growth Regulators In Seed Dormancy

     Gibberellins have been shown to be effective in breaking the dormancy in potato tubers and in tree buds in winter.

Senescence

     Like all organisms, plants grow old and die. Their life span may vary from a few years to few days. Even before the death of the whole plant, it is likely that a number of its organs and tissues have died earlier. As the young plant grows old, it undergoes ageing and develops into a mature plant in an orderly fashion. This phenomenon of deteriorative changes with ageing is called senescence.

Significance of Senescence

     Old and inefficient organs are replaced by young and developing organs. Leaf fall in deciduous trees reduces the rate of transpirational loss. This is an adaptation to tide over winter.

Plant Movements

     Movement in plants may not be very pronounced, but it is brought about by definite external and internal stimuli. Movement which occur due to external stimuli are known as induced or paratonic movements and the movements which occur due to factors inherent inside the plant body itself are known as autonomic or spontaneous movements.

Autonomic Movements

     It is the movement of the protoplasm. These are limited to aquatic plants and are free and spontaneous movements.

Induced or Paratonic Movements

     These movements are a result of the response to an external stimuli and the direction of movement is controlled by the direction of the stimulus.

Paratonic Nastic Movements

     Nastic movements do not depend on the direction of the stimulus and are diffused.

Process of Flowering

     Reproductive phase in a plant starts with the initiation of flowering. Some plants flower in a particular season only, some flower throughout the year. Plants like Agave, bamboo and certain palms bear flowers only once in their life time. Such plants are known as monocarpic. The stage of flower opening from a flower bud is known as anthesis.

Photoperiodism

     W. W. Garner and H. A. Allards work on Maryland Mammoth, a variety of tobacco and Glycine max established the fact that it was the length of the day, which controlled flowering in these plants.

Influence of Quality of Light on Flowering

     Green colour of the visible spectrum is ineffective in inducing flowering whereas blue colour induces poor flowering. The wavelength 580 nm to 680 nm in the red portion of the spectrum has been found to be most effective for inducing flowering.

Flowering Hormone

     There is increasing evidence to suggest that a flowering hormone exists in plants. This is has been supported by a number of grafting experiments. A short day plant kept in long day conditions can be induced to flower, if a properly photo induced plant is grafted on it. It clearly indicates that a diffusible flowering hormone has moved from one plant to the other and has induced the latter to flower. This hormone has been named florigen but it has so far not been isolated.

Summary

     Growth is one of the most fundamental characteristics of living organisms. It is accompanied by differentiation. The growth cycle of an annual monocarpic angiosperm begins with the zygote which undergoes a period of dormancy. Later, the dormant embryo develops into a seedling which grows into a vegetative phase and ultimately matures into the reproductive phase. The plant finally enters into the senescence stage which leads to death.



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