Animal responses
- taxis
- kinesis
- homing
- migration
Taxis |
Taxes is an innate locomotory behaviour. This means the animal's response is to instinctively move towards or away from a stimulus. Because this response involves moving in a particular direction (towards or away from), it is classed as a directional response.
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When an animal moves towards the stimuli, the response is termed 'positive'. When an animal moves away from the stimuli, the response is termed 'negative'.
Towards |
Away |
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Phototaxis |
Positive phototaxis - animal moves towards the light |
Negative phototaxis - animal moves away from light |
Chemotaxis |
Positive chemotaxis - animal moves towards the chemical |
Negative chemotaxis - animal moves away from the chemical |
Geotaxis / Gravitaxis |
Positive geotaxis - animal moves towards gravity |
Negative geotaxis - animal moves away from gravity |
Thermotaxis |
Positive thermotaxis - animal moves towards heat |
Negative thermotaxis - animal moves away from heat |
Thigmotaxis |
Positive thigmotaxis - animal moves towards source of physical contact |
Negative thigmotaxis - animal moves away from source of physical contact |
Rheotaxis |
Positive rheotaxis - animal travels againstthe current |
Negative rheotaxis - animal moves away from the current |
Applying these new terms
Below are three examples to get you started
- Research one example for each stimuli (i.e. one for chemotaxis - can be + or -) and explain how that response is advantageous to the organism.
- Link the advantage to survival and reproduction.
Below are three examples to get you started
In order for mating to occur, many species of female moths release pheromones (chemical scent) to attract a male of the same species. The response of the male can be termed positive chemotaxis - the male is moving towards the chemicals (pheromones)
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When earthworms encounter light, they display a negative phototaxic response. They move away from the light by quickly burrowing back down into the soil. This stops them from drying out and helps them avoid predators
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Mosquitoes, fleas and lice all display positive thermotaxic responses. They move towards sources of heat. As all these organisms are parasitic, this trait helps them to find a host
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Kinesis |
Kineses is also a innate locomotory behaviour (a movement response). However, the direction of movement is not the response to the stimuli - the direction in which the animal moves is completely random. Kineses is a non-directional response to a stimuli. With kineses, the rate of activity/movement is the response.
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There are two types of kineses:
Orthokinesis
The intensity of the stimulus determines the speed of movement/activity - the movement/activity of the animal is faster in unfavourable conditions and slower in favourable conditions This response means that in unfavourable conditions, the likelihood that the organism will find, and remain in, favourable conditions increases as it is moving faster to get out of those unfavourable conditions. When in favourable conditions, the organism is more likely to remain in those conditions as the speed of movement is a lot slower. |
Klinokinesis
The intensity of the stimulus determines the rate of turning - the animal's rate of turning is faster in unfavourable conditions and slower in favourable conditions This response means that in unfavourable conditions, the organism will have a greater chance of finding and returning to favourable conditions as the rate of turning has increased. When the organism is in favourable conditions and the rate of turning slows down, making it more likely the organism will remain in that environment. |
Applying these new terms
- Research one example for each response and explain how that response is advantageous to the organism.
- Link the advantage to survival and reproduction.
Homing |
Homing is when an animal is able to return to its 'home' from an unfamiliar location, or when there is unfamiliar territory is between the animal and its 'home'.
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Many different species of frogs and toads have a specific pond where they return every breeding season in order to lay their eggs. This is often the same pond where they themselves hatched.
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A honeybee's 'home' is its hive. During the day, many honeybees leave the hive and forage for food. They can reach as far as a kilometer away from their hive! Honeybees are able to find their way back to the hive every day.
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Salmon move between seawater into freshwater - they spend their adult lives in the ocean and return to the river where they were born in order to breed (spawn).
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Homing pigeons can return 'home' from many kilometers away - up to 2000km! They were used in WWI and WWII to deliver messages between bases.
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How do they do this?
Animals must be able to navigate their way home! To do this, they need:
Scroll down to see methods of navigation (they relate to both homing and migration).
Animals must be able to navigate their way home! To do this, they need:
- A sense of direction - i.e. an internal compass and/or internal clock - so the animal knows what way to go
- A sense of location - i.e. a way of knowing where they are - so they know approximately which way is home
Scroll down to see methods of navigation (they relate to both homing and migration).
Migration |
Migration an innate behaviour, and is when individuals move from one geographic location to another.
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Many animals have an annual return migration, where they spend part of the year in their breeding grounds (summer) and part of the year in their feeding grounds (winter), and travel in between those seasons. Some animals have a return migration at different stages of their life cycle, rather than an annual return migration. These animals will often return to their location of birth in order to have their offspring, and then return elsewhere to spend the majority of their adult life.
In order to migrate, animals need to be able to navigate, just like homing.
In order to migrate, animals need to be able to navigate, just like homing.
Methods of navigation
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Landmarks
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Sun compass
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Echolocation
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Chemical trails
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Star compass
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Magnetic fields
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Humans aren't the only organisms that use landmarks to know where they are, or where to go! This method of navigation is most common for animals that don't travel very far from home. Those animals who do may use coasts, mountain ranges and/or islands as landmarks to help them reach their destination.
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Many animals use the sun as a compass (using it to know where north and south are). However, animals must compensate for the sun's movement as the sun goes from E to W each day. During a long trip, animals must use their biological clock to keep track of time and adjust its orientation to the sun in order to get to its destination.
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This method of navigation is particularly common in migratory animals - they use a 'star compass' - much like how animals use the sun as a compass. These animals orientate themselves in relation to particular star patterns (constellations) and, like with the sun, must compensate for the movement of these patterns in the sky throughout the night.
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Many animals are able to use the Earth's magnetic fields as a navigation system. They have 'another sense' which can detect the direction and strength of the magnetic fields. Experiments have shown that when a magnet is attached to an animal that relies on this method of navigation, the magnet disrupts the magnetic fields and the animal was unsuccessful in navigating back 'home'. For migratory birds, they can determine their latitude using the direction of the magnetic fields. The strength of the magnetic fields varies from place to place and migratory birds can make a mental map of this and use it to help them navigate.
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Migration is different to homing as it is a very long journey which the animal must be very prepared for! An animal's biological clock is helpful to prepare an animal for migration, as it signals particular behaviours and physiological changes:
Migration is an innate behaviour, however individuals with more experience are more likely to be successful. This is because often the migratory animals use more than one method of navigation to ensure they are on the correct migratory path. Nearly all animals will remember landmarks during their migratory journeys, and if the animal was to be swept off their regular path for any reason, if they know the landmarks of the area they are more likely to re-orientate themselves and get to their destination safely. Learning landmarks is not innate, it is acquired over time!
- Storing a sufficient amount of fat - to ensure there will be sufficient energy supplies for the long journey head (as fat can be converted into energy)
- In birds, a moulting of feathers - so that old feathers are replaced with new feathers - particularly the flight feathers - to ensure no extra energy is used during the long flight due to having crappy feathers!
Migration is an innate behaviour, however individuals with more experience are more likely to be successful. This is because often the migratory animals use more than one method of navigation to ensure they are on the correct migratory path. Nearly all animals will remember landmarks during their migratory journeys, and if the animal was to be swept off their regular path for any reason, if they know the landmarks of the area they are more likely to re-orientate themselves and get to their destination safely. Learning landmarks is not innate, it is acquired over time!
How do animals know when to migrate?
Migratory animals who travel in particular seasons need to know what season it is! They work this out by comparing the day length (light hours) to their internal clock. This is done by the hypothalamus. The eyes contain photoreceptors (light receptor cells) which detect light. Day length is also termed photoperiod (referring to the period of light within the day - the 'day length'). When the photoperiod changes (i.e. days start getting shorter after the summer season) the hypothalamus picks up on this, and signals for the endocrine system to respond to this change in photoperiod. The animal's response is to begin preparing for migration. More on this in orientation in time.
Migratory animals who travel in particular seasons need to know what season it is! They work this out by comparing the day length (light hours) to their internal clock. This is done by the hypothalamus. The eyes contain photoreceptors (light receptor cells) which detect light. Day length is also termed photoperiod (referring to the period of light within the day - the 'day length'). When the photoperiod changes (i.e. days start getting shorter after the summer season) the hypothalamus picks up on this, and signals for the endocrine system to respond to this change in photoperiod. The animal's response is to begin preparing for migration. More on this in orientation in time.
Advantages of migration:
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Disadvantages of migration:
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Plant responses
- tropisms
- nastic responses
Plants are unable to get up and move, as they are stuck in the ground by their roots! However they are able to move different parts of their 'bodies' - such as their stems or leaves. They can really only do two types of 'movement' - they can grow in particular directions (tropisms) or they can change the turgidity of their cells (nastic response), causing their leaves to move.
Tropisms |
The directional growth response of plants to abiotic factors is termed 'tropsism'. The response occurs in attempt to place the plant in favourable environmental conditions and remove them from unfavourable environmental conditions. For example, plants will grow in the direction of higher light intensity rather than the direction of lower light intensity.
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Tropisms are a directional response - just like taxes are in animals - however as plants cannot get up and move in response to a stimuli, their directional response cannot be classed alongside animal directional responses. They can only grow toward or away from the stimuli. However like taxes, tropisms use the same prefixes to recognize the stimuli, and the response can be positive (towards) and negative (away from) the stimuli.
Towards |
Away |
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Phototropism |
Positive phototropism - plant grows towards the light |
Negative phototropism - plant grows away from the light |
Chemotropism |
Positive chemotropism - plant grows towards a chemical |
Negative chemotropism - plant grows away from a chemical |
Geotropism / Gravitropism |
Positive geotropism - plant grows towards gravity |
Negative geotropism - plant grows away from gravity |
Hydrotropism |
Positive hydrotropism - plant grows towards a source of water |
Negative hydrotropism - plant grows away from a source of water |
Thigmotropism |
Positive thigmotropism - plant grows up / on something else |
Negative thigmotropism - plant grows away from something else |
Applying these new terms
Below are three examples to get you started
- Research one example for each stimuli (i.e. one for chemotropism - can be + or -) and explain how that response is advantageous to the organism.
- Link the advantage to survival and reproduction.
Below are three examples to get you started
Germinating plant shoots demonstrate negative geotropism, as they grow away from the direction of gravity. This ensures the shoot can reach above the soil, where it can then display positive phototropism. In contrast, the first root (radicle) of the plant demonstrates positive geotropism and it becomes the taproot (main root) of the plant, pointing directly down and anchoring the plant into the soil.
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When pollen lands on the stigma of a flowering plant, positive chemotropism occurs. In order for fertilization to happen, the male gametes from the pollen must reach the ova which is inside the ovary. For this to happen, the pollen forms a pollen tube which grows down the stima towards the ovary. It grows in this direction as it is a response to chemicals released by the ovary. This response ensures that the male gametes reach the egg so that fertilization can occur.
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Many climbing plants, such as ivy and vines, grow all over a 'host' - this could be a plant or other structure. Positive thigmotropism allows a plant to grow up something, increasing its chances of obtaining as much light (and as much intense light) as possible for photosynthesis.
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How do plants grow?
Recap from Year 11
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Plants grow from the tips of their roots and shoots. The names of these regions are called the 'apical meristems' - a meristem is a place where mitosis occurs in plants! In these areas, cell division occurs and then the newly made cells absorb water into their vacuoles and elongate (get longer). They then undergo specialization which allows them to get a 'role' in the plant, or a 'job'. They may become xylem cells, phloem cells, cambium cells, palisade (leaf) cells... all sorts! This process is called primary growth - the growth of a plant up towards the light and down towards the soil, and where cells become differentiated. For a plant to become more structurally stable, and able to support all the weight of its leaves and branches, secondary growth must occur. This is how a plant gets thicker. It occurs in the cambium, which is a lateral meristem (meristem = mitosis). The cells produced here also differentiate into either xylem or phloem. As the plant matures, it can develop flowers which is how they can sexually reproduce. In doing so, they can form seeds which are dispersed and land somewhere. If the conditions are favourable, germination will occur and the cycle of growth starts again. |
Hormones involved
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The main type of auxin hormone is called indole acetic acid (IAA) It is produced in apical meristems (tips of roots and shoots) and promotes the:
* large amounts of auxin in the lateral buds (side stems) and roots can inhibit cell elongation/growth Produced in apical meristems (tips of roots and shoots) and have very similar effects to auxins - they promote the:
Produced in the tips of the roots ONLY - and travels up the xylem to reach the shoots and leaves and promotes:
ABSCISIC ACID (ABA) Produced in aging leaves and ripening fruit, and promotes:
Produced by aging fruit and aging leaves and promotes:
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Auxin and growth
Auxins are thought to be what are responsible for directional growth responses in plants. Research has focused on the coleoptile (sheath around the shoot tip) which is known to be receptive to both light and gravity. It is not clear what effect auxins have on other parts of the plants, such as roots, in terms of their directional growth response (tropism).
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In shoots:
In areas where there is lots of auxin present, there is lots of cell elongation. In areas where there is not much auxin present, there is not much cell elongation. If there is more auxin on one side of a stem compared to the other, the stem will bend because on one side there will be fewer elongated cells than the other! |
In roots:
In areas where there is lots of auxin present, there is little/no cell elongation. High levels of auxin inhibit cell elongation. However, in areas where there is a small amount of auxin present, there is cell elongation. If there is more auxin on one side of a root compared to the other, the root will bend because on one side there will be fewer elongated cells than the other! |
Phototropism
1. Stimulus is detected by the coleoptile cells at the tip of the shoot
- In leaves, the light-absorbing pigment is chlorophyll. In the tip of the shoot, there is a different light-absorbing pigment. It is called riboflavin (vitamin B12) and the absorption of light by this pigment is how the stimulus (light) is detected.
2. Some sort of message is transmitted from the coleoptile cells to the zone of elongation
- There are no 'concrete' facts about how this occurs but there are several conclusions that have been drawn about the signal - this is due to many different types of experiments being conducted on phototropism.
Conclusion 1: Phototropic response does not require cells to be intact - as when the tip was cut off and replaced, the response occurred as normal.
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Conclusion 2: The signal must be a water-soluble chemical, as when there was a piece of agar jelly put in between the tip and shoot, the response was normal - however when cocoa butter (water cannot move through) was put between the tip and shoot, there was no response.
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Conclusion 3: If light is not coming from directly above, but on an angle (i.e. sun rising or setting) signal only travels down the shaded side of the shoot - an experiment was conducted where a strip of mica (flexible type of mineral) was inserted into the shaded side of the shoot and the phototropic response did not occur as the pathway for the message was blocked by the mica.
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Conclusion 4: The shaded side is the side that elongates, the chemical message must stimulate cell elongation rather than division.
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3. The response occurs (elongation)
- Auxin (IAA) is transported from the tip of the shoot, the apex, to the shaded side of the coleoptile and further down the shoot to the zone of elongation. It is not yet understood exactly how auxin is transported. Scientists do know that it requires energy (as low temperatures and lack of oxygen inhibit it), it occurs more slowly than the transport of sugar through the phloem and it can only move in one direction - tip to base.
Auxin helps elongation because:
It allows the cell wall to become more 'stretchable' - allowing for easier uptake of water (osmosis) into the cell and the vacuole, allowing the cell to become enlarged. The cell wall more readily stretches upwards so that it is long (elongated) rather than stretching outwards.
To further complicate things (only slightly) - we need to incorporate leaves into the picture!
- Majority of the auxin produced by the shoots comes from the shoot apex. When both sides of a leafy plant are equally illuminated, the amount of auxin released from the apex and transported down the sides of the stem (to the elongation zone) are the same. This causes the plant to grow directly upright.
- However, when one side of the plant is exposed to more intense light (and thus is more 'illuninated') the leaves on the illuminated side of the plant export most of their auxin to the other side - the shaded side - where it accumulates. Auxin from the apex also travels down the shaded side to the zone of elongation. This drives the rate of cell elongation up, causing more elongation to happen on that side compared to the illuminated side. This causes the plant to 'bend' in that direction - it bends towards the light.
Geotropism
Geotropism is a response that is particularly important during the germination phase of plant growth. The radicle (first root) is positively geotropic and the plumule (first shoot) is negatively geotropic. The importance of these responses are quite simple - a seed just lands anywhere and in any direction. Roots need to go down and shoots need to go up - geotropic responses are the only way that both these things can happen!
Here are the details about how these two responses occur in the one seedling! Read the 'right side' before the 'left side' - I think it will make more sense that way!
Here are the details about how these two responses occur in the one seedling! Read the 'right side' before the 'left side' - I think it will make more sense that way!
Left side - positive geotropism
A plant's root cap is like a slimy little hat that the roots wear which helps them slide deeper down into the soil. The cells that make the slime are called root cap cells, and among them are some other specialized cells - called statocytes. These cells contain statoliths which are organelles that store lots of starch granules (stored glucose - and the roots convert the starch granules it into glucose (as required) for respiration and growth). Anyway, the starch in the statoliths weigh the statocytes down quite a bit, as these organelles are denser than the cytoplasm. Gravity acts on these dense little organelles and 'pulls' them down towards the soil. This leads to a redistribution of auxin within the plant's root(s) (remember auxin is produced here too!), and there ends up being heaps of auxin on the lower side of the radicle. This inhibits cell elongation on this side (too much auxin acts as an inhibitor) and therefore cell elongation can only occur where there is no inhibition - this is on the upper side of the radicle, causing the tip to point downwards. |
Right side - negative geotropism
Remember it isn't just the tips of the shoots that produce auxin, but the tips of the roots too! However, in a horizontal plumule, because of gravity - the auxin produced at the tip of the plumule sits on the lower side - rather than be evenly distributed throughout the plumule. This causes the cells on the lower side to elongate, causing the tip to point upwards. The direction of growth of the plumule is in the opposite direction to gravity, so is negative geotropism. |
You need to be able to like the following ideas together in a paragraph (or more):
- How the tropic response occurs
- Why the tropic response occurs
- The adaptive advantage to the plant for responding in this way
Nastic responses |
Nastic responses are comparable to the kineses responses of animals - both responses are non-directional responses. When plants undergo a nastic response, they are not moving any of their body parts towards or away from a stimulus - they are responding in a predictable manner to the intensity of the stimulus. Nastic responses are usually a change in a cell's turgidity - this can produce quite rapid leaf movement!
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Nastic responses are a plant's best form of 'movement' in response to their environment. These responses are much less understood compared to tropism responses. What is known about nastic responses is that they are quick and reversible movement responses. They are responses by parts of a plant (not the whole plant) to a change in abiotic factors. The aim of a nastic response is to remove that part of the plant from particular unfavourable environmental conditions in order to place it in more favourable conditions.
As nastic responses are not directional, they do not produce a 'positive' or 'negative' response.
As nastic responses are not directional, they do not produce a 'positive' or 'negative' response.
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Photonasty
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Nyctinasty
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Thigmonasty
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Thermonasty
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Other
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If a leaf is exposed to intense light for prolonged periods of time, the chloroplasts can become irreversibly damaged. As a response to high light intensity, plants can collapse their leaves to prevent damage to the structures within the leaf that assist with photosynthesis.
Is a specialised form of photonasty - it is when petals close or leaves droop in darkness and they open again in periods of light. This occurs in only certain species of flowering plants and occurs as a response to a change in photoperiod. Scientists don't yet know why plants do this, however they suggest that this 'behaviour' helps direct dew back into the soil.
There are some plant species who have leaves that are sensitive to touch - the response is that the leaves collapse inwards. Scientists think that this helps the plant by disturbing any potential herbivorous insects that may have been crawling on it (ready for a meal!). Venus fly traps display a thigmonastic response.
Thermonastic responses are responses to temperature - the flowers of some plants close in low temperatures to prevent damage to their reproductive structures.
There are also chemonastic, hydronastic and geonastic responses in plants!
How the response occurs:
At the base of the petiole are some specialized cells called pulvini. These cells are also found at the base of each leaflet in a leafy plant. Pulvini contain special motor cells (capable of rapidly responding to stimuli). When disturbed, the lower pulvini vells (holding the leaf/leaflet up) experience a sudden loss of turgor, as water leaves the cells.
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But how does water suddenly leave the cells?
When leaf cells are disturbed, they send a message to the pulvini cells which are the cells holding the leaf up. The message is a change in membrane potential which is similar to a nervous response in animals, however this is much slower. The pulvini cells receive the message and respond by pumping potassium ions out of the cytoplasm. This changes the water potential between the inside of the pulvini cells and their surroundings. As the water potential is lower outside the pulvini cells, water from the cells immediately moves out of the cells in order to even out the water concentrations. So basically a whole lot of water just leaves the lower pulvini cells after they pump out some potassium ions. This is the response! After the disturbance is removed (i.e. intense light, touch, etc) then turgor is restored to the cells who lost water and the leaves/leaflets can go back to their normal orientation. |
You need to be able to like the following ideas together in a paragraph (or more):
- How the nastic response occurs
- Why the nastic response occurs
- The adaptive advantage to the plant for responding in this way