FORM THREE BIOLOGY STUDY NOTES TOPIC 3: COORDINATION
Coordination in Plant, Concept of Tropic and Nastic Responses
The Concept of Tropic and Nastic Responses
Explain the concept of tropic and nastic responses
COORDINATION IN PLANTS
Both, plants and animals react (or respond) to various stimuli around them. But themethod of reacting to stimuli is not similar in plants and animals. They react to stimuli indifferent ways. For example, plants bend towards light but animals do not bend towardslight. The animal amoeba reacts to the presence of food by moving towards the foodparticle.
The plants do not have a nervous system and sense organs like eyes, ears, or nose, etc.,like the animals, but they can still sense things. The plants can sense the presence ofstimuli like light, gravity, chemicals, water, and touch, etc., and respond to them. Theysense these stimuli by using hormones in them.
The stimuli like light, gravity, chemicals, water, and touch, etc., are called environmental changes. So, we can also say that the plants coordinate their behaviour against environmental changes by using hormones. The hormones in plants do not act the sameway as in animals.
The hormones in plants coordinate their behaviour by affecting the growth of a plant.And the effect on growth of the plant can result in the movement of a part of the plantlike shoot (stem) or root, etc towards a certain stimulus.
Experiments to Investigate the Effects of Tropic and Nastic Responses in Plants
Carry out experiments to investigate the effects of tropic and nastic responses in plants
Plants receive and respond to a variety of stimuli that are important to their survival in the environment. These responses allow the plants to survive, grow, develop and reproduce.The movement of plants in the direction of stimulus is known as ‘tropism’. Tropisms aregrowth responses of plants that result in curvatures of plant organs toward or away fromcertain stimuli. Tropisms can be positive, in which case the plant will bend toward astimulus, or negative, in which case the plant will bend away from a stimulus.
The other movements shown by the plants are associated with the growth of the plants.For example, the shoot system moves towards sunlight and the root system towards earth.Thus, the plants also respond to their environment.
Important tropisms in plants include phototropism, geotropism, hydro tropism,chemo tropism and thigmotropism.
Phototropism is the tendency for plant organs to bend in response to a directional light source. For example, light streaming in a window from one direction will often cause the stems of plants placed nearby to bend toward the window, a positive phototropism.
Most plant shoots are positively phototropic because they tend to grow towards light.Most roots are negatively phototropic because they away from light.
The Importance of Tropic and Nastic Responses
Importance of phototropism
Phototropism is important to plants because of it enables the plant leaves to be placed under direct sunlight to absorb maximum light so as to carry out photosynthesiseffectively.
Geotropism is the movement of a part of the plant towards gravity. In most plants, roots-grow downward with gravity while shoots grow upward against gravity. Within hours,the shoot of a plant placed on its side will usually bend upward and the roots will benddownward as the plant reorients its direction of growth in response to gravity.
Most plant roots grow towards gravity and are said to be positively geotropic. Mostshoots grow away from gravity and are said be negatively geotropic.
Importance of geotropism
Geotropisms are important to plants because of the following reasons:
- It enables the plants to send roots into the ground hence anchoring the plant firmlyinto the soil.
- It enables plant roots to absorb water and mineral salts from the soil.
- Negative geotropism exhibited by the shoot enables the shoot to grow upwards, and in so doing, exposes the plant leaves to maximum sunlight for effective photosynthesis
Hydrotropism is the movement of a plant or part of a plant towards water. Plant roots normally grow towards moisture. They are therefore positively hydrotropic. If you plant a plant near a water source such as porous pot or river, the roots will always grow towardswater.
Importance of hydrotropism
It enables the plants to absorb dissolved minerals and water. Water is necessary forvarious functions such as:
- Numerous physiological reactions that take place within plant cells.
- Turgor pressure, which aids in plant support.
- Dissolution of mineral salts.
An example of positive and negative chemo-tropism is shown by a plant‘s roots; the roots grow towards useful minerals displaying positive chemo-tropism, and grow away fromharmful acids displaying negative chemo-tropism.
Chemotropism is the movement or growth of an organism or part of an organism in response to a chemical stimulus. During the process of fertilization the movement of pollen tube towards ovule due to secretion of a sugary chemical in the ovary is an example of chemotropism
Importance of chemotropism
- It enables the plant to absorb mineral salts from the soil when the roots grow towards beneficial chemicals such as fertilizers.
- Negative geotropism, such as when plant roots grow away from the toxins, enables the plant to survive by avoiding contact with such harmful chemicals.
- . It facilitates the fertilization process in flowering plants.
Thigmotropism or haptotropism
Thigmotropsim refers to non-directional movements which take place neither towards noraway from the stimulus. The best example of nastic movement is folding and drooping ofleaves of Mimosa pudica plant when its leaves are touched with fingers or any object.The leaves fold even when swayed about by wind. Also the specialized touch-sensitivetendrils of many vining plants, such as pea, will bend toward the side receiving a touchstimulus. Continual stimulation can lead to the coiling of the tendril around an object,which enables vining plants to grasp objects on which they can climb
Importance of thigmotropism
- Thigmotropism enables crawling plants to climb up higher plants and expose their leaves to sunlight for optimum photosynthesis.
- It enables the insectivorous plants such as the Venus flytrap to trap insects and digestthem to obtain nutrients.
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