Chemical Coordination in Plants Class 10 ICSE Notes
ICSE Class 10 Biology Chapter 7 Chemical Coordination in Plants Notes, Summary, Definition, Diagram. Chemical Coordination in Plants Notes.
→ Stimuli is a change in either external or internal environment of an organism.
→ Response is the resulting actions or movements caused by stimuli
→ Plant sensitivity and coordination is controlled by some chemicals produced by them.
→ Plants respond to stimuli by producing chemical compounds called “hormones” that work as messengers.
→ Because of some hormones plant growth is stimulated or maybe inhibited.
→ The main areas of growth (cell division) in plants are meristems as they are sensitive to hormones.
→ The hormone helps to stretch the cellulose wall of the meristematic cells to facilitate division.
→ Plants are sensitive to light and respond to the direction of light. Also sensitive to water, temperature, gravity, etc.
→ Shoot grows towards light but roots grow away from it.
→ Phytohormone – was coined to distinguish plant from animal hormones
Plant Hormones –
1.) Auxins –
→ First growth hormone discovered in plants
→ Are powerful growth-stimulants and also effective at extremely low concentration.
→ IIA (Indole-3-acetic acid) main natural auxin in plants.
→ Present in shoot apex, root apex, lateral meristems, etc
2.) Gibberellins –
→ Different forms of gibberellins from different plants – GA1, GA2, GA3 & so on.
→ Gibberellic acid (GA3) is most studied.
→ Promote growth of internodes by cell elongation
→ Present in meristematic region like stem-apex, root-apex, buds, seeds, etc.
3.) Cytokinins –
→ Has specific effects on cell-division
→ Stimulate plant growth by cell division as against auxin and gibberellins which stimulate growth by cell elongation.
→ Widely distributed in plants
→ Produced in root tips and transported through xylem
→ Found in germinating seeds, developing fruits, embryo etc.
4.) Ethylene –
→ Only hormone which is a gas at ordinary temperature.
→ Site of synthesis and site of action are not different.
→ Gas causing ripening of fruits
→ Produced in higher plants and fungi
→ More ethylene is produced in meristematic tissues.
5.) Abscisic acid (ABA) –
→ Growth-retarding hormone
→ Found in angiosperms, gymnosperms, pteridophytes and some mosses.
→ Found in chloroplasts of leaves
→ Fruits and seeds contain highest amount of ABA.
→ Also known as stress hormone
Plant hormone |
Site of synthesis |
functions |
1. Auxins | · Shoot apical meristems and young leaves | · Promote cell elongation
· Suppress the growth of lateral buds · Delay fall of leaves · Induce formation of parthenocarpic fruits |
2. Gibberellins | · Meristem of apical buds and roots | · Help in stem elongation
· Break dormancy of seeds and buds · Delay senescence · Induce parthenocarpy |
3. Cytokinins | · Roots and transported to other organs | · Stimulate cell division and cell enlargement
· Prevent ageing of plant parts · Inhibit apical dominance |
4. Ethylene | · Senescent leaves and flowers, germinating seeds and ripening fruits. | · Induces fruit ripening
· Promote senescence |
5. Abscisic acid | · Green fruits and seeds at the beginning of wintering periods. | · Induces dormancy of buds and seeds
· Inhibits seed germination and development · Stimulates closing of stomata |
Tropic movements in plants
→ The root must grow downwards into soil which will provide support, water and minerals for the plants.
→ Shoot must grow up towards the light source so that developing leaflets could carry out photosynthesis in order to produce food for the growing plant.
→ Movement of these parts of the plant take place in direct response to external stimuli.
→ The direction of response is related to direction from which stimulus comes. Such response is known as tropism.
→ Growth movements occurring in response to unidirectional external stimuli in a plant part are called tropic movements.
I.) Phototropism –
→ Movement towards light
→ Shoots are positively phototropic and roots are negatively phototropic
→ Auxins play an important role
→ Auxins help in bending of shoot towards the source of light by getting accumulated in the region of the shoot which is not facing light which result in rapid cell-elongation.
II.) Geotropism –
→ Growing towards the earth’s gravity
→ Also known as gravitropism.
→ Roots tip are positively geotropic and shoot tip is negatively geotropic.
III. Hydrotropism –
→ Movement of plant parts in response to water or moisture
→ When plant parts grow towards source of moisture called positively hydrotropic.
→ Roots are positively hydrotropic as water available in soil.
→ When plant part grow away from source of moisture is called negatively hydrotropic (shoot)
IV.) Thigmotropism
→ Growth movement of plant parts in response to touch stimulus
→ Plants such as sweet peas, cuscuta and vines have tendrils which coil around other plants in response to one sided touch.
→ Entire tendril becomes sensitive to stimulus.
→ Example of Thigmotropism is tendrils.
V.) Chemotropism –
→ Growth of plant organs in response to chemicals.
→ Example- movement of pollen-tube of angiosperm and gymnosperm towards sugars and peptones secreted by neck canal cells of female gametophyte
Glossary –
→ Parthenocarpy – natural or artificially induced production of fruit without fertilisation of ovules, which make fruits seedless.
Example – banana
→ Sunflower exhibit heliotropism in which sunflower head moves according to the sun (east to west direction). This is because sunflower contains auxin which is sensitive to sunlight.
Hence they migrate from the part of the plant bathed in sunlight to the shaded region of the stem.
Auxins stimulate the growth of shaded region of stem so that the flower end up bending towards the sun.
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Very good explanation