Living organisms depend upon their surrounding or environment. They respond and react to their environment. Conditions like chemical, thermal, electrical, photic, etc. in the environment always keep changing. To keep alive, organism musk keep on interacting with these changing environment conditions. This vital characteristic of organism is called irritability. It is poorly developed in plants due to their static mode of life. But, it is highly developed in animals, due to their mobile, active life.

Response to various environment factor (stimuli)such as light, cold, heat, pressure, etc., varies in different organisms. Every organism does not interect with every environmental change. A change to which an organism interacts is called a ‘stimlus’, while interaction is called its response. For example, plants bend toward light. Amoebae move towards food and tend to aggregate in moderately warm water. Protozoans like Amoeba and other avoid mechanical obstacles.

To respond properly to various stimuli organisms perform various physiological processes. Each physiological process requires regulation and control. They cannot run in an uncontrolled or wind manner. There occur two types of controls in organisms : nervous control and endocrine control.

1.      Nervous control : Nervous control is by sending electrical signals called nerve impulses. It is speedy and flexible, but effect is localized. Nervous control occurs by specialized cells called neurons or nerve cells and nerve fibers. Neurons respond to stimuli. A nurone is the structural and functional unit of nervous system. Neurons system, which includes brain, spinal cord and nerves, receives information from its surroundings, processes and interprets it, and responds accordingly. This system also passes information from one internal system to another. For example, smell or taste of food cause salivation. This system, thus, controls ans coordinates various body functions.

Multicellular animals excepts sponges, have neurons, for responding to stimuli. Most plants do not have any special structure for responding to external stimuli.

2.      Endocrine control : Endocrine control is by sending chemical messengers called hormones. Hormones and secretions of endocrine glands. This control is usually slow acting and its effects.



In plants, the control system is not well developed and elaborate as in animals. They also do not possess brain and structure like neurons. However, plants responds to various external stimuli like light, temperature, gravitational force, touch, chemicals, etc. Growth , development and movements in plants are regulated by both internal and external factors. The functions of control and coordination is performed by certain certain chemical substances known as plants hormones or phytohormones or growth hormones.

Phytohormones are of following types:-

       I.            Auxins

    II.            Gibberellins

 III.            Cytokinins

 IV.            Ethylene

    V.            Abscisic Acid (ABA).

Auxins: These are the best known plants hormones Charles Darwin was the first to report the presence of growth substances in plants. F.W. Went (1928) gave the term auxin for the growth promoting substances present in plants.

Some views regarding the mode of action of auxins are :

1.      Increase in cell wall plasticity which bring about cell elongation.

2.      Increase of mRNA synthesis from DNA which makes enzymes for cell elongation.

3.      Increase in the rate of oxidative phosphorylation which result in increase energy supply.

Various physiological effects of auxin on plants are :

1.      Promotion of apical dominance of terminal bug.

2.      Stimulation of cell division and cell growth.

3.      Induction of roof formations in stem cutting.

4.      Prevention of immature leaf fall.

5.      Formation of seedless fruit.

6.      Regulation of plants growth movements.

7.      Increase in the seed rate of plants cell respiration.

8.      Induction of flowering in some plants like pineapple.

9.      Prevention of premature fruits fall.

10.  Prevention of lodging of tall plants.

Gibberellins : Very first gibbrellin was isolated from a fungus named Gibberella and was named as GA3. After that a large number of different gibbrellins have been isolated from different group of palnts. Applications of gibbrellins shows following different physiological effects on plants groeth and development.

1.      Breakage of dormancy and thus helps in seed germination.

2.      Conversion of a dwarf plants into a tall one. A cabbage plants when treated with Ga3, it becomes 6-8 feet tall.

3.      Flowering of long day plants under short day conditions when treated with gibbrellin.

4.      Low temperature treatment required in some plants for flowering can be substituted by application of Gibbrellins.

5.      Rapid root growth in cucumber and lettuce.

6.      Induction of parthenocarpy in some fruits like tomato, apple, cucurbita etc.

7.      Promotions of formation of male flowering in female plants.

8.      Formation of large sized fruits of grapes and tomoto.

9.      Delay in the ripening of fruits of orange.

Cytokinins : Phytohormones of this group regulate cell devision in plants. They are produced in activity growing plant tissues such as root tips and developing embryos. Miller (1955) discovered the cytokinin in coconut milk and named it as Kinetin. Application of cytokinins produce following physiological effects in plants.

1.      They help in breaking seed dormancy.

2.      Delay in ageing of plants.

3.      Cytokinin treatment induces flowering in long day plants under short day conditions.

4.      Activation of growing of lateral buds of plants into branches.

5.      Cytokinin along with auxin, induce organ formation in plants.

6.      Their application also brings about resistance to diseases.

7.      They produce elongation of cell.

8.      Promotion of female flowers.

9.      Promotion of formation of interfascicular cambium.

Ethylene : It is the only gaseous phytohormones present in very small amount in plants. This hormones is mainly related to ripening of fruits. Ethylene is a natural .

Various physiological effects of ethylene on plants are as follows.

1.      Promotion of premature fall of leaves , flowers and fruits.

2.      Increase in ageing of plants parts.

3.      Ripening of fruits is include by it.

4.      Induction of flowering in pineapple and mangoes.

5.      It decrease the rate of longitudinal growth but increase radial growth.

6.      Induction of positive geotropic movements in roots.

7.      It helps to break dormancy in seeds in some plants.

Abscisis Acid (ABA) : It is naturally occurring growth inhibitor found in plants. Dormant potato tubers are rich in ABA. It was first isolated from mature cotton fruits. It inhabits protein synthesis in plants. It is translocated through xylem and phloem both. It acts as na antagonist to gibbrelline.

It has following physiological effects on plants:

1.      Includes seed dormancy and bud dormancy.

2.      Promotion of premature fall of leaves, flowering and fruits.

3.      Promotion of ageing in plant parts.

4.      Closure of stomata, thereby reducing the rate of transpiration.

5.      Suppression of seed generations.

6.      Initiation of flowering in short day plants under long day conditions.

7.      Increase resistance of plants towards cold.

8.      Delay in cell division and cell elongation.

9.      Inhibition of synthesis of enzymes amylase needed for seed germination.

Effects of light and temperature on plants : Light play an important role on growth and development of plants. Some plants need for exposure of light for a time-period longer than the critical period for flowering. These are known as long day plants. Plants, those requires light for a shorter period than their critical period for flowering are called short day plants. The remaining ones are known as day or intermediate day plants. They can produce flowering both in long and short day conditions.

The ability of plants to respond to daily periods of light and dark is known as photoperiodism. Special pigment called phytochrome is responsible for such photoperiodic response. It is present in very small amount in plants.

Temperature also make cobsiderable effects on growth of plants especially  flowering. Plants growing in temperature regions need a low temperature periods before their flowering occurs. As application of temperature between 1degree- 10 degree celcius to winter varities of wheat, cotton, etc. increase the growth rate of their seeding and includes early flowering. This effects of low temperature treatment on flowering is known as vernalization.

Life period of plants between its complele maturity and final death is known as senescence. There is gradual deterioration in structure and function of plants during this period.

Senescence is present in all plants. It may be senescence of whole plant or a part of palnt. There is high rate of catabolic activities during this process and it is controlled by some growth hormones.

Plant shed their leaves, flower, and fruits. Shedding of these plants parts is due to change in their hormonal balance. Abscisic acid play an important role in this process knowm as abscission.



Unlike plants, control and coordination in animals is performed by both neurons and harmones. As already mentioned, multicellular animal except sponges have specialized cell called neurons, to respond to astimulus. Spongs are fixed like plants and donot possess neurons. Cnidarians, such as Hydra, possesses nerve cell which form a nerve net in the body. They also possess sense organs such as ocelli and statocyst. This enables them to behave as a coordination unit and makes them more efficient than the sponges.

In certain animals nerve nets condense into nerve mass called ganglion. Ganglion and nerve cord from the nervous system in higher invertebrates. In flatworms, like liver flake nervous system consist of concertration of nervous tissue into ladder-like form. It compromise brain and two main longitudinal nerve cords connected as intervals by transverse commissures. A variety of sense organ occur in free living forms.

Higher invertebrates like insects, earthwarm and other possess a well developed nervous system ansd sense organs. They have a bilobed nerve mass, ventral, ganglionated nerve cord and nerves arising from these ganglia.

Animals possess specialized structure called sense organ for receving a variety of external informations.

Receptors are :

(i)                 Photoreceptors for light, e.g., eye.

(ii)               Phonoreceptors for sound, e.g., ears.

(iii)             Gustatoreceptors for taste, e.g., tongue.

(iv)             Olfactoreceptors for smell, e.g., nose.

(v)               Tangoreceptores for touch, e.g., skin.

Receptors os sense organs stimuli and pass information to barin by a part of neurons called sensory. Neurons. Informations or stimuli are interpreted in brain. Then, another type of neurons, called motor neurons, transmit information from brain to the effector organ. Muscles and glands act as effector organs. Effector organs then respond accordingly. Sense organs cannot analyse information received by them. Center for analysis of information is the brain. For example, information regarding vision, hearing, smell, movement, action, etc. are analyse by brain and not by receptors.

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