There are two types of reproduction.
Sexual reproduction
Involves many processes; meiosis, pollination, fertilization, seed production, seed dispersal and germination. The plant uses gametes to make offspring and this creates genetic variation.
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Asexual reproduction
Involves the parent plant undergoing mitosis to basically 'grow' another plant off itself. There are no gametes involved, therefore no pollination, fertilization, seeds or genetic variation.
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It's important to remember that most plants have the capacity to undergo both types of reproduction - they are not mutually exclusive. However, the underlying concepts of how each occur need to be separated in your minds. This is so you have the ability to compare and contrast the two methods.
There are 6 major events
in sexual reproduction
Meiosis |
Pollination |
Fertilization |
Seed development |
Seed dispersal |
Germination |
Cells in the anther and ovary undergoing two divisions to produce haploid gametes - pollen and ova. |
The process of getting the pollen (male gamete) to the ova (female gamete) so that fertilization can occur. |
When nucleus of pollen and ova fuse inside the ovary - creates a zygote which turns into a seed (containing embryo). |
When a flower is fertilized, many changes occur. The flower turns into a fruit and the seeds form within the fruit. |
The process whereby seeds are relocated to (hopefully) ideal conditions for germination. |
The process of a plant emerging from its seed. First the roots, then the shoots. The plant version of a bird hatching from its egg! |
1. Meiosis
You do not have to go into detail with this (it is more of a genetics concept). What you need to know is:
Meiosis is what allows there to be genetic variatiom within the plant population, which is really important if environmental conditions are not exactly the same every day. In the diagram on the right, the cell at the top is the original cell. The 4 cells at the bottom are 4 gametes (i.e. pollen or ovules). See how they have half the number of chromosomes compared to the original cell? |
2. Pollination
Pollination is the movement of pollen from its point of origin (the anther) to the stigma. There are two 'types' of pollination:
Self pollination |
Cross pollination |
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Both these types of pollination can be facilitated by either the wind, or by animals (birds, insects, etc).
- Plants will be adapted to one of these two methods of pollination - animals or wind - in order to have a high chance of successful fertilization.
Wind vs. animal pollination
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Animal-pollinated flowers
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Wind-pollinated flowers
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Animal-pollinated flower and pollen adaptations
Lots of plants are dependent on pollinators. Pollinators are vectors which transport pollen directly from anther to stigma. Plants that are dependent on animals for pollination often have the following adaptations:
Flower adaptations - think about how each helps pollination to be successful! |
Nectary present - full of sweet sugary goodness that attracts birds, bats and insects to the flowers - the nectary is found behind the stamen (anther+filament) so in order to get to the good stuff, the animal will unknowingly brush past the pollen and pick it up. Once the pollinator gets its feed of sugar it will go to another flower in search for more, again brushing past the stamen but also the stigma of the flower, delivering the pollen it picked up. Talk about door to door service! |
Anther + stigma found within petals - hidden from the wind - insects must pass them in order to get to nectary so they will inevitably pick up some pollen. If the stamen dangled outside the petals then the pollinator might not brush past it. |
Colourful petals and sweet scent - again, this adds some 'attraction factor' so particular pollinators pick this flower over that flower. |
Larger petals - petals often act as a 'landing pad' for smaller pollinators. Being able to hold the weight of something that is going to help you produce offspring is quite advantageous! If your petal was too small your pollinator might slide off and not deliver your pollen to a different plant. |
Sticky stigma - the stickiness will also come in handy when the pollen reaches the stigma of another flower, as it will be able to stick to the stigma without dropping off! |
Pollen adaptations - think about how each helps pollination to be successful! |
Less pollen granules - due to the 'door to door service' provided by pollinators, there is no point making heaps of pollen only for a lot of it to go to waste. This uses up valuable energy that could be spent making petals larger or making nectar! Odds are most of the pollen produced is going to end up in the right place so there's no benefit in making heaps of it for the 'off chance' that some of it gets lost along the way. |
Sticky/rough pollen granules - pollen grains need to be able to attach themselves to their pollinator, so having sticky and/or rough is advantageous to you if you're an animal-pollinated flower. Pollen that is transported via animals is often a bit larger than pollen transported by the wind. |
Wind-pollinated flower and pollen adaptations
Many plants are instead dependent on wind for pollination. When a gust of wind passes by the plant, it picks up the pollen and takes it away, hopefully to another plant of the same species in order for fertilization to happen! Plants that are dependent on the wind for pollination often have the following adaptations:
Flower adaptations - think about how each helps pollination to be successful! |
No nectary present - wind isn't attracted to sweet sugary goodness so the plant would be wasting energy making nectar if no-one was going to eat it - instead it can focus on making lots more pollen! |
Anther + stigma found outside of petals - it would be too hard for the wind to pick up pollen from inside the petals, so the filament is long and the anther sticks out beyond them. The same with the stigma - it would be too hard for pollen floating in the wind to land inside some petals (it would just land on the petals) so the stigma dangles outside them. |
Dull petals and no scent - again, no point using energy to make colourful petals and nice smells as animals don't pollinate these flowers. Pollination is likely to be more successful if the plant spent that energy on making heaps of pollen, rather than colourful petals. |
Smaller petals - no need for a landing pad - save the energy and use it for something more beneficial (like more pollen!) |
Feathery stigma - having a feathery stigma increases the surface area (space) in which pollen granules can land - more space means more pollen granules could land on the stigma, increasing the chance of a successful fertilization! |
Pollen adaptations - think about how each helps pollination to be successful! |
More pollen granules - the chances of the wind whisking up the pollen and delivering it straight to the stamen of another plant is not very high, so plants that are pollinated by the wind use most of their energy on making lots of pollen to increase their chances of success! |
Small + smooth pollen granules - smaller and smoother pollen also means it is light - this makes it easier for the wind to pick it up off the anther and allows it to be transported further. |
Why don't plants just self-pollinate? Isn't it easier and faster?
Yes, but...
Yes, but...
Self-pollination results in offspring that are very genetically similar to the parent plant and this means there wouldn't be much variation within the population if plants were pollinating and fertilizing themselves. Having a small amount of genetic variation is not good for the population - if there was to be an environmental change that one plant couldn't cope with, it is likely the others won't be able to either as they are so similar! The population would be vulnerable to dying out. Plants have ways to combat self-pollination and the resulting self-fertilization:
Although most plants have both male and female parts (making them hermaphrodites) they have adaptations to prevent self-pollination.
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3. Fertilization
Fertilization occurs when the pollen and ovule fuse, creating a zygote! In particular, the nuclei of the pollen cell fuses with the nuclei of the ovule cell. To the two nuclei join together, allowing the chromosomes from each cell to come together and form homologous pairs (a genetics concept).
Pollen and ovules are both haploid cells (n) - they contain half of the genetic material from their parent plants.
When the pollen and ovule come together a zygote is formed (2n) (n (pollen) + n (ovule) = 2n).
Pollen and ovules are both haploid cells (n) - they contain half of the genetic material from their parent plants.
When the pollen and ovule come together a zygote is formed (2n) (n (pollen) + n (ovule) = 2n).
Never ever say that fertilization is when the pollen fertilizes the ovule.
- When you use the word you're trying to define in its own definition, you still aren't actually stating what it means.
- Try and use the word fuse in your definition, instead of fertilize.
Steps of fertilization:
1. Pollen grains land on stigma (either because an animal delivered it or the wind blew it there)
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2. Pollen grain bursts open (this is stimulated by chemicals on stigma) pollen tube begins to grow out of pollen grain - it grows down towards the ovary (towards chemical signal created by ovules)
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3. Pollen tube enters ovule and male nuclei (from inside pollen) fuses with nuclei of ovule - homologous chromosomes match up! This means there are the right number of chromosomes to allow a seed to develop.
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4. The product of fertilization is a zygote - which will undergo mitosis and grow into an embryo (which is safely inside a seed).
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4. Seed development
Once fertilization occurs and a zygote is formed, many changes start to happen to the flower. Just like when a woman becomes pregnant (i.e. her egg is fertilized by a sperm to form a zygote) her body changes to prepare for the offspring.
Once fertilized, a flower basically falls apart and a fruit is formed from the remains. A fruit can look like what we normally picture in our heads (i.e. an apple), or it can become very hard. When it becomes hard, it is a nut, like almonds and peanuts. Whether a fertilized flower forms a fleshy fruit or a hard nut is dictated by the species of plant and how the seeds are dispersed. This is elaborated on in the next section.
Once fertilized, a flower basically falls apart and a fruit is formed from the remains. A fruit can look like what we normally picture in our heads (i.e. an apple), or it can become very hard. When it becomes hard, it is a nut, like almonds and peanuts. Whether a fertilized flower forms a fleshy fruit or a hard nut is dictated by the species of plant and how the seeds are dispersed. This is elaborated on in the next section.
Depending on their dispersal methods, different things can happen to seeds in order for them to be dispersed effectively! With fruit such as apples, the petals, carpel, stamens etc. drop off the flower and the ovary swells. The ovary swells so much and becomes fleshy - this creates the fruit. The fruit is a delicious meal for an animal, and by eating the fruit that animal disperses the seeds. If the seeds get dispersed by wind, the ovary dries out instead, creating a lightweight and aerodynamic pocket in which the seed(s) is/are found loosely attached. This way they can blow off in the wind!
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Below: tomato development (yes, it's a fruit!). The yellow flower below will undergo all the changes pictured once it has been fertilized.
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Seed structure
This is important - make sure you are familiar with all the parts of a seed.
Testa |
Radicle |
Plumule |
Cotyledon |
Endosperm |
Micropyle |
The seed coat - a protective layer between the embryo and the environment! "I'm testing the coats!" |
The first root that emerges from the embryo. It grows down into the soil in search of water and nutrients "R for roots, R for radicle." |
The first shoot that emerges from the embryo. It grows up towards the sky in search of sunlight. The 'real' leaves (not the cotyledon) grow from the shoots. The P looks like a little leaf above the ground! |
The 'food package' found stored in a seed. It contains lots of starch which is stored glucose (glucose molecules joined together makes starch). |
Acts as a food store (best to think of it like a cotyledon) for the developing plant embryo. The endosperm is only found in monocots "Endo, mono, both have 4 letters?" |
The little hole in the seed where water and oxygen can enter the plant. Water, to start germination. Oxygen, for respiration. "Micro-hole." |
Please note:
The radicle and plumule are joined together - they're part of the same body. Just like your head and legs are part of the same body.
The body is called the embryo - this is the part that actually grows into the plant. The rest of the seed (cotyledons, testa, etc) do not become a new plant - they are only there to help make the conditions ideal for the embryo to develop and germinate.
The radicle and plumule are joined together - they're part of the same body. Just like your head and legs are part of the same body.
The body is called the embryo - this is the part that actually grows into the plant. The rest of the seed (cotyledons, testa, etc) do not become a new plant - they are only there to help make the conditions ideal for the embryo to develop and germinate.
5. Seed dispersal
Seeds aren't just found in fleshy fruit, the fruit can also dry out and become a nut. A nut is actually a hard fruit with a shell and a seed. Crazy! Seeds have different adaptations - which are related to the many ways of being dispersed - not just by being eaten.
Seed dispersal the spreading of seeds out over a greater area to reduce competition between the seed and its parent plant. Seed dispersal is important because if a seed was dropped right next to its parent plant, they would both be competing for:
water, nutrients (from in the soil), space, sunlight.
Seed dispersal the spreading of seeds out over a greater area to reduce competition between the seed and its parent plant. Seed dispersal is important because if a seed was dropped right next to its parent plant, they would both be competing for:
water, nutrients (from in the soil), space, sunlight.
Why do plants need these things?
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Sunlight
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Water
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Nutrients
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Space
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Plants use the energy found in sunlight (radiant energy) in order to power the process of photosynthesis. Without sunlight, this process will not occur and glucose (food) will not be made for the plant. Without glucose, the plant cannot build new structures or use it for respiration to get energy for growth.
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Plants need water for photosynthesis as well - it is a reactant that, along with carbon dioxide, gets turned into glucose. Without water, plants would also be unable keep their cells turgid (nice and full) which causes the plant to wilt. A wilted plant (leaves flopping down) is not very good at photosynthesizing as the leaves are not exposed to the light!
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Nutrients such as nitrogen and calcium are required for proper functioning of all the cells in the plant. They also help to build new plant structures (new cells). Plants need around 17 different nutrients for normal growth! We often deliver these to plants via their fertilizer or potting mix.
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Space is crucial to a plant. More space means more room for leaves to grow. Leaves are the food factory where photosynthesis occurs - more leaves means more photosynthesis, which results in more glucose produced. More glucose leads to more respiration, making more ATP and therefore allowing rapid growth!
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Timing of flowering and seed dispersal -
Flowers don't just leave themselves open for pollination all the time - they do it at particular times of the year. There is a reason for this! It is so that when the seed is dispersed, it can land and successfully start to grow (germination). If the conditions aren't right (too cold, too wet, etc) then the seed may die before it can grow into a plant. To be energy-efficient, plants will only flower at particular times of the year (you'll learn how they know how to do this in L3 Bio!) so that pollination is more likely to be successful (as all plants of the same species will flower around the same time), plus the offspring of these plants will have favourable conditions to grow in and will be more likely to survive!
Methods of seed dispersal
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Wind
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Water
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Animals
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Bursting
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Adaptations of wind-dispersed seeds:
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Advantages -
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Disadvantages -
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Advantages -
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Disadvantages -
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Eaten by animals:
Adaptations of animal-dispersed seeds (via. eating):
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Advantages -
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Disadvantages -
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Carried by animals:
Advantages -
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Disadvantages -
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Advantages -
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Disadvantages -
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6. Germination
The process by which a plant grows from a seed! It starts when water enters the seed, and stops when the seedling plant is able to photosynthesize and sustain its own energy needs.
The germination process:
1. Water enters the seed through the micropyle. This kick-starts germination.
Seeds can last a long time without water and still survive, as they are dormant until water causes the physical changes which lead to germination. |
2. Oxygen also enters the seed through the micropyle. In order for the plant to create ATP, oxygen is required in conjunction with glucose. It is needed for respiration! No growth can occur without that process happening.
3. The radicle (first root) emerges and grows downwards to anchor the seed. The radicle also allows water to begin to be absorbed from the environment and transported into the seed itself, as well as minerals found in the soil. 4. The plumule (first shoot) emerges and grows upwards towards the light. The first leaves start to emerge. When they are above the ground they are able to start conducting photosynthesis. 5. Germination is completed as soon as the plant can photosynthesize enough to support itself and its energy requirements. Stored starch in the cotyledon is used as a source of glucose for respiration until photosynthesis allows glucose to be made and then used in respiration instead. |
Conditions required for successful germination:
Water
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OxygenIs required for respiration (aerobic) - the transformation of glucose and oxygen into ATP (energy), carbon dioxide and water (waste products).
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Warmth
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