Irritability or sensitivity is the power to respond to a stimulus. A stimulus can be defined as any change in external and internal environmental conditions which can bring about a change in the activity of the whole or part of the organism. Response is the term used for the change in the activity of the organism. The behaviour of an organism is therefore greatly affected by the responses it makes to both external ( outside the body) and internal ( within the body) stimuli. This ability to respond to stimulus resides in the protoplasm and is a characteristic of all living organisms i.e plants and animals. Plants react slowly to stimuli, the only exceptions being a few motile plants and parts of plants, e.g gametes while animals respond almost instantaneously to external stimuli. Responses to internal stimuli may be slow or fairly rapid in animals.
The external stimuli that brings about response in organisms are light, temperature, gravity, touch, water and chemical substances. Examples of internal stimuli are changes in metabolic conditions, disease condition, sex urges and parental influences. In most animals, well defined systems are concerned with the perception and transmission of stimuli. These are the nervous and hormonal systems which contain the receptors of the stimuli. In plants, however there are usually no specialized systems which deal with the perception and transmission of stimuli. The protoplasm of young cells at the shoot and root apices usually acts as a receptor of external stimuli. The response made by an organism is generally shown by movements. Either the whole organism moves from one place to another, as it happens in animals and motile plants, or some part of it's body moves either towards or away from the stimulus, as in plant shoots and roots, and the limbs of animals.
IRRITABILITY IN PLANTS
Plant movements in response to internal stimuli are known as spontaneous movementswhile those in response to external stimuli are known as induced, or irritable movements. Spontaneous movement are seen in the streaming of protoplasm within the cell and in the rotating or hoisting movements of the growing tips of shoots and roots. Induced movements include nastic, tactic and tropic movements.
NASTIC MOVEMENTS IN PLANTS
These are non directional movements made by a part of a stationery plant in response to an external stimulus which is usually describe as diffuse. The closing up of the leaves of a mimosa plant on being touched, the closing up of the lid of a pitcher plant on the entry of an insect or the sleeping movement of some compound leaves such as those of the flamboyant in response to low light intensity and temperature are examples of the responses shown by parts of certain plants to external stimuli. Nastic movement may be carried out in different ways. The folding of the leaves of certain plants e.g the mimosa is brought about by changes in the water content of the cells. When the water content falls, loss of water from the cells into the large intercellular spaces occurs. Consequently, the cells shrink and the leaves fold up. On the other hand, when the water content rises the cells become turgid and the leaves resume their original upright positions.
TACTIC MOVEMENT IN PLANTS
When a whole organism moves from one place to another in response to external stimuli such as light, gravity, water and certain chemicals, it is said to exhibit a tactic movement, or taxis. Chlamydomonas and Euglena show tactic movements. Usually motile plants swim towards light. Such a movement is described as phototatic. A movement in response to the presence of a certain chemical in the environment is described as chemotatic. The male gametes of seaweeds are saud to exhibit a chemotatic response when they are attracted by some chemicals secreted by female sex organs and swim towards thOrganisms that move towards a stimulus are said to show positive response , while those that move away from the stimulus are said to grow negative response. Therefore, the source or direction of the stimulus has a direct relationship to the direction of movement of the organism. This type of response to the stimulus is said to be directional.
A culture of motile algae such as chlamydomonas or volvox is used. A certain part of the culture tube is covered with black paper and the tube is placed near a window. Most of the algae can be seen to aggregate near the part of the tube which is not covered with black paper. Hence, they demonstrate a positive phostotatic response. If a light source of high intensity is placed near the tube, the algae move away to the covered part of the tube. This is because a high light intensity evokes a negative phototatic responseTROPIC MOVEMENTS IN PLANTS
When the fixed part of a stationary plant moves in response to a stimulus, the reaction is described as a tropism or tropic movement. The stimulus is usually applied from one direction and the response is also made in one definite direction. The response may be towards the stimulus (positive) or away from it (negative). The stimulus is, therefore, unilateral (from one direction) and the response is directional.
Tropisms are growth movement which takes place at a very slow pace. The growth movement is caused by an increased or decreased rate of growth on the side of the organ which is under the influence of the stimulus, with respect to the opposite side. This results in a growth curvature. Growth curvatures usually occur when the outer surface of a root touches an object such as a large stone. In response to the stimulus of touch, the cells of the root which come in contact with the stimulus grow more rapidly than those at the other side, as a result of which the root turns away from the object. Thus, a negative tropic curvature occurs when the plant organ grows on the side nearer the stimulus so that it bends away from the stimulus. In the case of a positive tropic curvature, the side of the organ further from the stimulus grows more rapidly so that the organ bends towards the stimulus..em.,
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