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Here is a term paper on ‘Plant Hormones’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Plant Hormones’ especially written for school and college students.
Term Paper on Plant Hormones
Term Paper Contents:
- Term Paper on the Introduction to Plant Hormones
- Term Paper on the Hormones and Regulation of Plant Growth
- Term Paper on Gibberellins
- Term Paper on Cytokinins
- Term Paper on Abscisic Acid
- Term Paper on the Hormonal Control of Flowering
- Term Paper on Plants and Photoperiodism
- Term Paper on the Circadian Rhythms
- Term Paper on Changing the Rhythms
- Term Paper on the Clock Functions
- Term Paper on the Touch Responses in Plants
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1. Term Paper on the Introduction to Plant Hormones:
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As a plant grows it does far more than increase its mass and volume. It differentiates, forming a variety of cells, tissues, and organs, and undergoes morphogenesis, taking on the shape characteristic of the adult sporophyte.
Moreover, many of its activities are finely tuned to its environment and to the changing pattern of the seasons. Many of the details of how these processes are regulated are not known, but it is clear that plant development and growth depend on the interplay of a number of internal and external factors.
Chief among the internal factors are the plant hormones. Hormones, by definition, are substances that are produced in one tissue and transported to another, where they exert highly specific effects. Hormones help the plant integrate the growth, development, and metabolic activities of its various tissues.
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Typically they are active in very small quantities. In the shoot of a pineapple plant, for example, only 6 micrograms of auxin, a common growth hormone, are found per kilogram of plant material. One enterprising plant physiologist calculated that the weight of the hormone in relation to 1 kilogram of shoot is comparable to the weight of a needle in a 22-ton haystack.
The term “hormone” comes from the Greek word meaning “to excite.” It is now clear, however, that many hormones have inhibitory influences. So, rather than thinking of hormones as stimulators, it is perhaps more useful to consider them as chemical messengers. But this term also needs qualification. The response to the particular “message” depends not only on its content but also upon how it is “read” by its recipient. All known plant hormones have a multiplicity of effects.
2. Term Paper on the Hormones and Regulation of Plant Growth:
The Auxins:
The first plant hormones to be isolated were the auxins. The effects of these hormones were observed by Charles Darwin and his son Francis and first reported in The Power of Movement in Plants, published in 1881. The Darwins were studying the bending toward light, or phototropism, of grass seedlings.
They noted that the bending takes place below the tip, in the lower part of the coleoptile. Then they showed that if they covered just the terminal portion of the coleoptile with a cylinder of metal foil or a hollow tube of glass blackened with India ink and exposed the plant to a light coming from the side, the characteristic bending of the seedling did not occur.
If, however, the tip was enclosed in a transparent glass tube, bending occurred normally. Bending also occurred normally when the lightproof cylinder was placed below the tip. “We must therefore conclude,” they stated, “that when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the material to bend.”
In 1926, the Dutch plant physiologist Frits W. Went succeeded in separating this “influence” from the plants that produced it. Went, cut off the coleoptile tips from a, number of oat seedlings. He placed the tips on a slice of agar (a gelatin like substance), with their cut surfaces in contact with the agar, and left them there for about an hour.
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He then cut the agar into small blocks and placed a block off-center on each stump of the decapitated plants, which were kept in the dark during the entire experiment. Within one hour, he observed a distinct bending away from the side on which the agar block was placed.
Agar blocks that had not been exposed to a coleoptile tip produced either no bending or only a slight bending toward the side on which the block had been placed. Agar blocks that had been exposed to a section of coleoptile lower on the shoot produced no physiological effect.
Went interpreted these experiments as showing that the coleoptile tip exerted its effects by means of a chemical stimulus (in short, a hormone) rather than a physical stimulus, such as an electric impulse.
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In response to this chemical stimulus, the side of the coleoptile adjacent to the agar block grew more rapidly, causing the coleoptile to bend away from that side. The hormone was named auxin, a term coined by Went from the Greek word auxein, “to increase.”
Several different substances with auxin activity have now been isolated from plant tissues, and others have been synthesized in the laboratory; they are all known as auxins. The most common of the natural auxins is indoleacetic acid, abbreviated IAA. One of the synthetic auxins, known as 2, 4-D, is commonly used as an herbicide (like many other physiologically active compounds, auxins are toxic in high doses). For reasons not known, 2, 4-D and related compounds are effective on broad-leaved plants at concentrations not harmful to grasses and so are commonly used on lawns to control broad-leaved weeds.
Auxins, in massive doses, were among the herbicides used in Southeast Asia during United States’ participation in the Vietnam War. The synthetic auxins, unlike IAA, are not readily broken down by natural plant enzymes or by bacteria.
Their longer effective life makes them better suited for commercial purposes. Their destructive effects on vegetation are long-lasting.
The phototropism observed by the Darwins results from the fact that under the influence of light, auxins migrate from the light side to the dark side of the tip.
The cells on the dark side, having more auxin, elongate more rapidly than those on the light side, causing the plant to bend toward the light. Auxin effects on stems and coleoptiles can be detected within 15 minutes of application of the hormone, suggesting that auxin exerts its effects by producing changes in the permeability of the cell membrane.
The action spectrum for phototropism is not the same as that for photosynthesis; only light in the blue region of the spectrum (less than 500 nanometers in wavelength) produces the phototropic response.
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Various plant tissues show other responses to auxin. Shoots elongate in response to auxins produced in the meristem. Apparently, under the influence of auxin, the cell wall becomes more plastic and thus the cell takes up more water and elongates. Because of the construction of the cell walls, expansion of the cell is unidirectional.
When large concentrations of auxins are present, the growth of the main roots is inhibited, and this inhibition is presumed responsible for the capacity of seedlings to orient themselves in the ground. In low concentrations, auxins induce the formation of adventitious roots; rooting preparations used by gardeners contain auxin.
In most dicot species, the growth of lateral buds is inhibited by auxins. If you pinch off the growing tip (meristem) of the stem of the houseplant Coleus, for example, the lateral buds begin to grow vigorously, producing a plant with a bushier, more compact body.
If you treat the “eyes” (actually lateral buds) of a potato with auxin, they will be inhibited from sprouting and so the tubers can be stored longer. The formation of the abscission layer has been correlated with diminished production of auxin in the leaf. Auxins are also involved in the growth of fruits.
Mechanism of Action:
Auxin increases the plasticity of the cell wall. This effect is brought about by a complex series of interactions that are not yet fully understood.
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During continued growth, auxin stimulates specific RNA and protein biosynthesis, and under the control of this newly forming protein, certain of the old bonds holding the ceil wall together are broken as new carbohydrate material is incorporated into the structure.
However, response to auxin by coleoptiles, is very rapid, occurring in too short a time for new protein synthesis to be initiated. Current studies indicate that, in these reactions, auxin exerts its effects by binding to the cell membrane and changing its permeability. Auxin responses are associated with a rapid movement of hydrogen ions across the membrane.
3. Term Paper on Gibberellins:
The gibberellins were first discovered by a Japanese scientist who was studying a disease of rice plants called “foolish seedling disease.” The diseased plants grew rapidly but were spindly and tended to fall over under the weight of the developing seeds. The cause of the symptoms, it was found, was a chemical produced by a fungus, Gibberella fujikuroi, which infected the seedlings. The substance, which was named gibberellin, was subsequently isolated not only from the fungus but from many species of plants. These hormones are produced in apical meristems, leaves, and plant embryos.
The most remarkable results are seen when gibberellins are applied to plants that are genetic dwarfs. Under gibberellin treatment, these dwarfs become indistinguishable from normal tall plants.
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Some plants-lettuce and cabbage are common examples-first grow as rosettes; the leaves develop but the internodes do not elongate until just before flowering. At that time the stem elongates rapidly, a phenomenon known as bolting.
In the case of cabbage and other biennials, bolting and flowering do not normally occur until after a period of cold; however, bolting and flowering can be induced by the application of gibberellin.
Gibberellins have also been shown to play a role in the growth of plant embryos and seedlings. In grass seeds, there is a specialised layer of cells, the aleurone layer, just inside the seed coat. These cells are rich in protein. During the early stages of germination, the embryo produces gibberellin, which diffuses to the aleurone layer. In response to the gibberellin, the aleurone cells produce enzymes that hydrolyse the starch, proteins, and other storage products to soluble sugars, amino acids, and other small molecules that the embryo and then the seedling uses for its growth as it pushes up through the soil.
Recent studies by Joseph Varner of Washington University show that, in the germinating barley grain, gibberellin acts directly at the nuclear level, activating formerly repressed genes and so resulting in new messenger RNA formation.
4. Term Paper on Cytokinins:
The cytokinins are a group of hormones originally detected in coconut “milk,” which is a liquid endosperm. They were found to promote the division of plant cells isolated in test tubes, and their name is derived from cytokinesis, meaning cell division. They have now been found in numerous plants, largely in actively dividing tissues, including germinating seeds, fruits, and roots.
Responses to Cytokinin and Auxin Combinations:
Studies of responses to combinations of auxin and cytokinins are helping physiologists understand how plant hormones work to produce the total growth pattern of the plant.
Apparently, the undifferentiated plant cell-such as the meristematic cell-has two courses open to it- Either it can enlarge, divide, enlarge, and divide again, or it can elongate without cell division. The cell that divides repeatedly remains essentially undifferentiated, or embryonic, whereas the elongating cell tends to differentiate and become specialised.
In studies of tobacco stem callus, the addition of an auxin to the tissue culture produced rapid cell expansion, so that giant cells were formed. A cytokinin alone had little or no effect. Auxin plus cytokinin resulted in rapid cell division, so that large numbers of relatively small cells were formed.
By slight alterations in the relative concentrations of auxin and cytokinin, investigators have been able to affect the development of undifferentiated cells growing in tissue culture. When a high concentration of auxin is present, undifferentiated tissue gives rise to organised roots.
With higher concentrations of cytokinins, buds appear. However, lest you think this is simple, we shall describe another tissue culture study, in which tuber tissue of the Jerusalem artichoke was used. In this study, it was shown that a third substance, the calcium ion, can modify the action of the auxin-cytokinin combination.
Auxin plus low concentrations of cytokinin was shown to favour cell enlargement, but as calcium ion was added to the culture, there was a steady shift in the growth pattern from cell enlargement to cell division.
High concentrations of calcium ion apparently prevent the cell wall from expanding, and at such concentrations the cell switches course and divides. Thus, not only do hormones modify the effects of hormones, but these combined effects may be, in turn, modified by non-hormonal factors, such as calcium ions and, undoubtedly, many others.
5. Term Paper on Abscisic Acid:
Soon after the discovery of the growth-promoting hormones, plant physiologists began to speculate that growth-inhibiting hormones would be found, since it is clearly advantageous to the plant not to grow at certain times and in certain seasons.
Not long afterward, an inhibitory hormone was isolated from dormant buds. Subsequently, the same hormone was discovered in cotton bolls (the fruit of the cotton plant), where it was found to promote abscission. The hormone is called abscisic acid.
Abscisic acid accelerates the dropping of leaves and of fruit; the presence of auxin inhibits abscission. Application of abscisic acid to vegetative buds changes them to winter buds by converting the outermost leaf primordia to bud scales; these inhibitory effects can be overcome by gibberellin. The appearance of hydrolyzing enzymes induced by gibberellin in barley seeds is inhibited by abscisic acid. Abscisic acid, though it has little effect on dwarf plants, reduces the growth of normal plants; this inhibition can be counteracted by gibberellin. Kinetin inhibits yellowing in excised leaves; abscisic acid causes green leaves to yellow.
In short, abscisic acid, in many of its effects, serves as an inhibitor of the growth-promoting effects of other hormones, emphasizing the concept that growth is the result of a balance of different factors.
Finally, as if the subject of stomatal movements was not already sufficiently complex, abscisic acid has been shown to be a potent effector of stomatal closure.
When, leaves lose moisture, abscisic acid synthesis increases and the stomata close. A variety of tomato is known, called a “wilty mutant,” that cannot make abscisic acid and cannot close its stomata.
Ethylene:
Ethylene is an unusual growth regulator in that it is a gas, a simple hydrocarbon, H2C = CH2. Its effects have been known for a long time. In the early 1900s, many fruit growers made a practice of improving the colour and flavor of citrus fruits by “curing” them in a room with a kerosene stove. (Long before this, the Chinese used to ripen fruits in rooms where incense was being burned.)
It was long believed that it was the heat that ripened the fruits. Ambitious fruit growers, who went to the expense of installing more modern heating equipment, found to their sorrow that this was not the case.
As experiments showed, it was actually the incomplete combustion products of kerosene that were responsible for ripening the fruits. The most active gas was identified as ethylene. As little as 1 part per million of ethylene in the air will speed the ripening process.
Subsequently, it was found that ethylene is produced by plants, as well as by kerosene stoves, that it appears just before and also during fruit ripening, and that it is responsible for a number of changes in colour, texture, and chemical composition that take place as fruits mature.
Auxin at certain concentrations causes a burst of ethylene production in some plants. It is now believed that some of the effects on fruits and flowers generally attributed to auxin are related to the release of ethylene.
Similarly, it is now believed that abscisic acid is not the direct cause of abscission in most cases; rather, it is hypothesized, abscisic acid induces ethylene formation, which accelerates senescence, .which stimulates abscission.
6. Term Paper on the Hormonal Control of Flowering:
Flowering also appears to be under the control of a hormone or hormones, although to date no specific one has been isolated and identified. Some of the earliest experiments on this hypothetical flowering hormone were carried out by a Russian scientist, M. H. Chailakhyan, in the 1930s.
Working with a species of chrysanthemum, Chailakhyan found that if the upper portion of the stem was stripped of its leaves and the leaves on the lower stem were exposed to an appropriate light cycle, the plant would flower, a phenomenon known as photo-induction.
If, however, only the upper, leafless stem and its buds were exposed, no flowering occurred. He interpreted these results as indicating that the leaves form a hormone that moves to the apical meristem of the plant and initiates flowering. He named this hypothetical hormone florigen, the “flower maker.”
Subsequent experiments showed that the flowering response does not take place if the leaf is removed immediately after photo induction. But if the leaf is left on the plant for a few hours after the induction cycle is complete, it can then be removed without stopping flowering.
The flowering hormone can pass through a graft from a photo induced plant to a non-induced plant. If a branch is girdled, florigen movement ceases. This led to the conclusion that florigen moves by way of the phloem system, the means by which most organic substances are transported.
However, despite this strong evidence for the existence of florigen, the hormone has never been isolated and, also, more recent evidence indicates that other, inhibitory factors are also involved in flowering.
7. Term Paper on Plants and Photoperiodism:
In many regions of the biosphere, the most important environmental changes affecting plants (and indeed, land organisms, in general) are those that result from the changing seasons.
Plants are able to accommodate themselves to these changes because of their capacity to sense and, more important, to anticipate the yearly calendar of events-the first frost, the spring rains, long dry periods, long growing spells, and even the time that nearby plants of the same species will be in flower.
For many plants, all of these determinations are made in the same way- by measuring the relative periods of light and darkness. This phenomenon is known as photoperiodism.
Photoperiodism and Flowering:
The effects of photoperiodism on flowering are particularly striking. Plants are of three general types-day-neutral, short-day, and long-day. Day-neutral plants flower without regard to day length. Short-day plants flower in early spring or fall; they must have a light period shorter than a critical length-for instance, the cocklebur flowers when exposed to 15½ hours or less of light.
Other short-day plants are poinsettias, strawberries, primroses, ragweed, and some chrysanthemums. Long-day plants, which flower chiefly in the summer, will flower only if the light periods are longer than a critical length. Spinach, potatoes, clover, henbane, and lettuce are examples of long-day plants.
The discovery of photoperiodism explained some puzzling data about the distribution of common plants. Why, for example, is there no ragweed in northern Maine? The answer, investigators found, is that ragweed starts producing flowers when the day is less than 14½ hours long.
The long summer days do not shorten to 14½ hours in northern Maine until August, and then there is not enough time for ragweed seed to mature before the frost. For similar reasons, spinach cannot produce seeds in the tropics. Spinach needs 14 hours of light a day for a period of at least two weeks in order to flower, and days are not this long in the tropics.
Note that the cocklebur and spinach will both bloom if exposed to 14 hours of daylight, yet one is designated as short-day and one as long- day. The important factor is not the absolute length of the photoperiod but rather whether it is longer or shorter than a particular critical interval for that variety.
And in some varieties, 5 or 10 minutes’ difference in exposure can determine whether or not a plant will flower.
These early field studies on photoperiodism in plants were carried out by W. W. Garner, H. A. Allard, and co-workers at the U.S. Department of Agriculture, known as the Beltsville group, for the small town in Maryland where they carried out their studies.
Photoperiodism has now been demonstrated in many species of insects, fish, birds, and mammals. It influences such diverse phenomena as the metamorphosis from caterpillar to butterfly, sexual behaviour, migration, molting, and seasonal changes in coat or plumage.
Measuring the Dark:
Following the early studies by the Beltsville group, other investigators, Karl C. Hamner and James Bonner, began a laboratory study of photoperiodism. They also used the cocklebur as the experimental organism. The cocklebur is a short-day plant, requiring 15½ hours or less of light per 24-hour cycle to flower.
It is particularly useful for experimental purposes because a single exposure under laboratory conditions to a short-day cycle will induce flowering two weeks later, even if the plant is immediately returned to long-day conditions.
Also, the cocklebur can withstand a good deal of rough treatment. With the cocklebur it was possible to demonstrate that flowering will not occur if the leaves are removed but will occur if even a portion of one leaf remains on the plant. In other words, it is the leaf that “perceives” the light, and this function can, in some cases, be assumed by a fraction of the total leaf surface.
In the course of these studies, in which they tested a variety of experimental conditions, the investigators made a crucial and totally unexpected discovery.
If the period of darkness is interrupted by as little as a 1-minute exposure to the light of a 25-watt bulb, flowering does not occur. Interruption of the light period by darkness has no effect whatsoever on flowering.
Subsequent experiments with other short-day plants showed that they, too, required periods not of uninterrupted light but of uninterrupted darkness.
What about long-day plants? They also measure darkness. A long- day plant that will flower if it is kept in a laboratory in which there is light for 16 hours and dark for 8 hours will also flower on 8 hours of light and 16 hours of dark if the dark is interrupted by even a brief exposure to light.
A. Lang of Michigan State University has carried out studies involving grafts of day-neutral, long-day, and short-day plants that strongly indicate the presence of a hormone or hormones other than the hypothetical florigen.
For instance, he and his co-workers have shown that if the long- day plant Hyoscyamus niger is completely defoliated, flower buds will form irrespective of photoperiod; however, if even one leaf remains, it will flower only on long days.
If a day-neutral plant is grafted to a long-day plant, flowering of the day-neutral plant is accelerated by exposure to long days. Exposure to short days, however, inhibits flowering of a day-neutral plant grafted to a long-day plant. In short, flower-inhibitors as well as flower-promoters are involved.
Photoperiodism and Phytochrome:
Following up on the clues from the Hamner and Bonner experiments, the Beltsville group was able to detect and eventually to isolate the pigment involved in photoperiodism. This pigment, which they called phytochrome, exists in two different forms- One is P660 and one is P730.
The numbers refer to the wavelengths of light the two forms absorb; 660 nanometers is red light and 730 nanometers is far-red. P660 absorbs red light and is converted to P730, which is the active form. This conversion takes place in daylight or in incandescent light; in both of these types of light, red wavelengths predominate over far-red. When P730 absorbs far-red light, it is converted back to P660. The P730 to P660 conversion can also take place in the dark, which is how it usually occurs in nature. P730, the active form of the pigment, promotes flowering in long-day plants and inhibits flowering in short-day plants.
Other Phytochrome Responses:
Many types of small seeds, such as lettuce, germinate only when they are in loose soil, near the surface. Otherwise the seedling would have little chance of reaching the light. Red light, a sign that sunlight is present, stimulates seed germination by converting phytochrome to the active form.
Similarly, phytochrome is involved in the early development of seedlings. When a seedling develops in the dark, as it normally does underground, the stem elongates rapidly, pushing the shoot up through the soil layers. Any seedling grown in the dark will be elongated and spindly with small leaves.
It will also be almost colourless, because the chloroplasts do not synthesize chlorophyll until they are exposed to light. Such a seedling is said to be etiolated. When the seedling tip reaches the light, normal growth begins. Phytochrome is involved in the switching of etiolated to normal growth.
If a dark-grown bean seedling is exposed to only one minute of red (660 nanometers) light, it will respond with normal growth. If, however, the exposure to red light is followed by a one-minute exposure to far- red light, etiolated growth continues, thus negating the original exposure. The exposures can be alternated repeatedly and the plant responds only to the last one perceived.
The way in which phytochrome acts, is not known. One recent suggestion is that it alters the permeability of the cell membrane, permitting particular substances to enter the cell, or, perhaps, inhibiting their entry, and that these substances, which probably include hormones, regulate the cell’s activities.
8. Term Paper on the Circadian Rhythms:
Some species of plants have flowers that open in the morning and close at dusk or they spread their leaves in the sunlight and fold them toward the stem at night. As long ago as 1729, the French scientist Jean-Jacques de Mairan noticed that these diurnal (daily) movements continue even when the plants are kept in dim light.
More recent studies have shown that less evident activities, such as photosynthesis, auxin production, and the rate of cell division, also have daily rhythms. The rhythms continue even when all environmental conditions are kept constant. These regular day-night cycles have come to be called circadian rhythms, from the Latin words circa, meaning “about,” and dies, “day.” Circadian rhythms now have been found throughout the plant and animal kingdoms.
Biological Clocks:
Are these rhythms internal-that is, caused by factors within the plant or animal itself-or is the organism keeping itself in tune with some external factor? For a number of years, biologists debated whether it might not be some environmental force, such as cosmic rays, the magnetic field of the earth, or the earth’s rotation, that was setting the rhythms.
Attempts to settle this recurrent controversy have led to numerous experiments under an extraordinary variety of conditions. Organisms have been taken down into salt mines, shipped to the South Pole, flown halfway around the world in airplanes, and, most recently, orbited in satellites.
Although there is still a vocal minority that believes that circadian rhythms are under the influence of a subtle geophysical factor, most workers now agree that the rhythms are endogenous-that is, they originate within the organism. Strong evidence in support of this belief is that the rhythms are not exact.
Different species and different individuals of the same species often have slightly different, but consistent, rhythms, often as much as an hour or two longer or shorter than 24 hours. Nothing, however, is known about the physical or chemical nature of this internal timing device, which is often referred to as a biological clock.
Another important feature of these rhythms is that they do not speed up as the temperature rises, as you might expect since enzymatic activities must be involved and enzyme reactions take place more rapidly as temperatures increase.
Therefore, the clock must contain within its workings some type of compensatory mechanism-a feedback system that adjusts it to temperature changes. This idea is supported by the observation that in some organism circadian rhythms seem to slow down as the temperature rises, rather than speed up, suggesting that overcompensation may be taking place.
9. Term Paper on Changing the Rhythms:
Although circadian rhythms probably originate within the organisms themselves, they can be modified by external conditions-a fact that is, of course, important to the survival of both individuals and species. For instance, a plant whose natural daily rhythm shows a peak every 26 hours when grown under continuous dim light can adjust its rhythm to 14 hours of light and 10 hours of darkness.
It can also adjust to 11 hours of light and 11 of dark (or 22 hours). Such adjustment to an externally imposed rhythm is known as entrainment. If the new rhythm is too far removed from the original one, however, the organism will “escape” the entrained rhythm and revert to its natural one.
A plant that has been kept on an artificial or forced rhythm, even for a long period of time, will revert to its normal internal period when returned to continuous dim light.
10. Term Paper on the Clock Functions:
Biological clocks are believed to play an essential role in many aspects of plant and animal physiology. For instance, insects are more active in the early evening hours. Bats that feed on insects begin to fly each evening just when the insects are most available.
Moreover, caged bats under controlled and constant laboratory conditions continue to show this sort of activity about every 24 hours, indicating that they are following an internal rhythm, not merely responding to environmental cues.
Some plants secrete nectar or perfume at certain specific times of the day. As a result, insects-which have their own biological clocks- become programmed to visit these flowers at these times, thereby ensuring maximum rewards for both the insects and the flowers.
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The ability to tell time also appears to be involved in a number of complex and fascinating phenomena such as the extraordinary ability of migrating birds and turtles to navigate.
Biological clocks enable organisms to recognise the changing seasons of the year by “comparing” external rhythms of the environment, such as changes in day length, to their own relatively constant internal rhythms. This capacity is an important factor in regulating the growth cycle of many plants.
Finally, biological clocks are clearly involved in photoperiodism. In order for organisms to detect changes in day length-and remember that in some cases, they are accurate to within 15 minutes-they must have some constant, a clock, with which to compare them. The chemical nature of the biological clock-or indeed if there is just one kind of clock or many-is still not known.
11. Term Paper on the Touch Responses in Plants:
Many plants respond to touch. One of the most common examples is seen in tendrils or winding stems. They wrap around objects with which they come in contact and so enable the plant to cling and climb. The response can be rapid; a tendril may wrap around a support in less than a minute.
Cells touching the support shrink slightly and those on the other side elongate. There is some evidence that auxin plays a role in this response.
A more spectacular response is seen in the sensitive plant, Mimosa pudica, in which the leaflets and sometimes entire leaves droop suddenly when touched. This response is a result of a sudden change in turgor pressure in cells at the base of the leaflets and leaves.
The change in turgor pressure appears to involve the movement of potassium ions, as do the changes in stomata, but again, how the change is triggered is unknown. There is also some controversy about its survival value to the plant.
Mimosa pudica often grows in dry, exposed areas where it may be subjected to drying winds; strong winds may shake the leaves enough to make them fold up, so conserving water. Another suggestion is that the wilting response makes the plant unattractive to grazing animals or startles chewing insects.
The triggering of turgor changes by touch is also involved in the capture of prey by the carnivorous Venus flytrap. The leaves of the Venus flytrap are hinged in the middle, and each leaf half is equipped with three sensitive hairs.
When an insect walks on one of these leaves, attracted by the nectar on the leaf surface, it brushes against the hairs, triggering the trap like closing of the leaf. The toothed edges mesh, the leaf halves gradually squeeze closed, and the insect is pressed against digestive glands on the inner surface of the trap.
The trapping mechanism is so specialised that it can distinguish between living prey and inanimate objects, such as pebbles and small sticks that fall on its leaves by chance- The leaf will not close unless two of its hairs are touched in succession or one hair is touched twice.
Studies have recently been made by investigators at Washington University in St. Louis on how the sundew traps insects. The club- shaped leaves of the plant are covered with tiny tentacles.
A sticky droplet surrounding the tip of each tentacle attracts insects. When an insect is caught on the tip of a tentacle, the tentacle bends in rapidly, carrying the prey to the center of the leaf, where it is digested by enzymes. Micro electrodes placed in the tentacles have revealed that the rapid response is accompanied by an electric impulse that moves down the tentacle. This impulse is similar, in principle, to the action potential that forms the basis of the nerve impulse in animals.
The investigators predict that electric signals will be found to coordinate a variety of functions in plants.
