Plants need energy to grow.
Plants get their energy via a process called respiration. This is when glucose and oxygen undergo a reaction to produce carbon dioxide, water and ATP (energy). Plants undergo respiration in their cells to give those cells energy to undergo mitosis (making more cells, which is what growth is)!
So basically, plants need glucose to be able to grow. So where does it come from?
In a seed
Glucose is stored as starch in the seed's cotyledon. When the enzymes inside the seed are activated by water, they start converting the starch back into glucose to be used in respiration. Different enzymes then use the glucose and oxygen to reslease the energy (ATP) that was locked up in the glucose - this process is respiration! The energy from this can then be used in the apical meristem to fuel mitosis.
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In a plant that has germinated
A plant that's germinated is a plant that is able to produce its own glucose via photosynthesis (rather than use the stored stuff in its cotyledon - as it is no longer a seed so it has no cotyledon!). So, these more 'grown up' plants undergo photosynthesis to make glucose! That glucose can then, straight away, be used for respiration to release energy, and then the energy can be used by the plant to grow.
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Types of growth
Both can happen at exactly the same time in a plant!
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Primary growth
What does primary growth give to a plant?
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Height
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Depth
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Reach
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More height means increased access to light
- This leads to more photosynthesis, more glucose and consequently more energy for growth and reproduction.
More depth means more water
- This allows for more photosynthesis (as water is a reactant) and water also fills cell vacuoles, helping leaves 'stick out' to get sunlight. As water enters the roots, minerals from the soil also enters with them which benefits plant growth.
- the deeper the roots are in the ground, the less likely the plant will topple over in the wind etc.
More reach means more directional growth outwards
- so the plant takes up more space with its branches, so more space for leaves etc. which means more photosynthesis (glucose, respiration, energy, growth!).
How does primary growth occur?
You really need to draw this out to fully understand the process.
There are three steps in primary growth:
1 - Mitosis, 2 - Elongation, 3 - Specialization/differentiation Follow the blue cells all the way along 1. Mitosis
Mitosis occurs in the meristem of plants, and is where cells divide to produce identical cells. For plants to grow, more and more and more cells need to be made! That is what growth is, literally more cells being created, making the plant bigger in size. For plants to increase in height, depth, reach and width, mitosis must occur.
Primary growth always occurs at the very top for the shoots, which is why when people carve their names into trees the carving never moves up the tree, the tree just keeps growing from the top! Another way of thinking of it is like how you build sandcastles - more sand is always added to the top - you don't lift the whole sandcastle up to add sand to the bottom. |
2. Elongation
Elongation is the next stage, where the cells that have been created by mitosis absorb water. This swells the cells, making their shape longer (elongation). Cells must undergo this change in order for specialization to occur.
Water is held in the cell's vacuole, so as water enters the plant it enters the vacuole and fills it up, causing the cell to get a little bit bigger. |
3. Differentiation
Once cells are elongated they are now able to become specialized. This means they get their job to do, depending on where they are found. Cells in the shoots (above ground) can become xylem, phloem, cambium, cells that make up a leaf, etc. Cells in the roots (below ground) can be xylem, phloem and cambium too, as well as root hair cells and root tip cells. Below is a cross section of a stem (as if you chopped the stem and looked straight down it) showing vascular bundles that have resulted from PG.
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Below, if you follow the blue cells, shows primary growth in its entirety.
First we have mitosis, producing new identical cells. This is happening continuously. Once the cells have been produced, they elongate. This happens in the 'zone of elongation' (below where mitosis is occurring). Once the cells have elongated they are now ready to differentiate (become different cells), or specialize. This occurs in the 'zone of differentiation'. Once cells have their role, they may begin undergoing the particular cell processes required to fulfill this role.
First we have mitosis, producing new identical cells. This is happening continuously. Once the cells have been produced, they elongate. This happens in the 'zone of elongation' (below where mitosis is occurring). Once the cells have elongated they are now ready to differentiate (become different cells), or specialize. This occurs in the 'zone of differentiation'. Once cells have their role, they may begin undergoing the particular cell processes required to fulfill this role.
The result of primary growth is specialized cells that can now fulfill their role so that the plant can live, grow and reproduce!
What can cells specialize into?
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Secondary growth
What does secondary growth give to a plant?
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Primary growth hasn't stopped!
Height, depth and reach continue to increase. |
Secondary growth can only occur in dicotyledons, as the first part of SG is all the cambiums joining up together in a ring. In dicots, the vascular bundles (containing the cambium) are situated in a ring, so this is possible (on right side of picture). However, the vascular bundles of monocots are situated randomly throughout the stem (left side) and not in a ring, therefore the cambium from the bundles cannot join together. This makes secondary growth impossible for monocots, which is why they are always quite small plants.
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How does secondary growth occur?
Vascular bundles join together to form one ring within the stem. This occurs by:
- The cambium (lateral meristem) continuously undergoing mitosis outwards and eventually the cambium from each vascular bundle merges with that of the neighboring bundle.
- The cambium, in its one big ring, undergoes mitosis like normal, with resulting cells specializing into xylem and phloem. Note: xylem is always on the inside of the cambium and phloem is always on the outside of the cambium. This is important. The way this works is that when new cells are produced, if those cells are located on the inside of the cambium they will specialize into xylem. Eventually there will be a ring of xylem and phloem on either side of the cambium. Instead of being called xylem and phloem, as they are a result of secondary growth, they are termed secondary xylem and secondary phloem.
- This takes years, not days! It is a long process. This is why it takes quite a while for trees to grow big!
Cambium doesn't stop doing mitosis, and continuously produces new cells between the secondary xylem and secondary phloem. These new cells replace old cells, they just adding another layer. Eventually, the most inner xylem cells will die and become wood. The new cells that end up on the outer-side of the cambium become phloem. Eventually, the outer most phloem cells die and become cork/bark.
This keeps happening for the entirety of a tree's life. The creation of secondary xylem also is how trees get their growth rings (this is how people tell the age of a tree). |
Why is making more xylem and phloem so important?
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Xylem
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Phloem
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Water is essential for a plant to live, photosynthesize, grow and reproduce. Water enters the plant through the roots and then goes straight into the xylem. Inside the xylem vessel is a suction force, sucking up the water from the roots and taking them up the stem and to the leaves where it can be used.
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Phloem is essential for a plant as it is how the glucose gets around to all the different parts of a plant! Respiration needs to occur close to where the energy is going to be used (i.e. apical meristem) so it needs to be taken from where it is made (in the leaves) and transported up/down the stem.
Features of phloem cells:
The phloem vessel is a lot simpler compared to xylem.
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Factors that affect growth
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Temperature
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Light intensity
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Oxygen availability
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Water availability
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- Low temperatures = not much energy for enzymes to work effectively = slower reactions (such as respiration which is required to make energy for growth).
- High temperatures = can denature enzymes if too hot = no reactions.
- Warm temperatures = optimum temperature for enzymes to work, so high rate of growth.
- High light intensity = lots of photosynthesis = lots of glucose (food) made - plenty available for respiration = makes ATP (energy) which can then be used in growth!
- Poor light intensity = less photosynthesis, less food made, less can be used in respiration so less energy made, less growth as a result!
- Oxygen needed for aerobic respiration, so if there is poor oxygen availability there will be less respiration, less energy made and therefore less available to fuel primary growth!
- Sufficient oxygen means there'll be sufficient respiration (given all other requirements are available).
- Water is required for photosynthesis so if there's a lack of water there'll be a lack of glucose and consequently energy for the plant to undergo growth.
- Water activates the enzymes in a seed so if there is a lack of water the rate of primary growth during germination will be low.
- Water also transports nutrients so with a lack of water there will also be a lack of nutrients available, affecting the rate of growth.
Growth rings
- actually seeing the effect temperature, light intensity and water has on a plant's growth.
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Think of it this way...
Growth is basically a plant making lots more cells. More cells makes an organism bigger which is growth!
So keeping that in mind... think about how they relate to the seasons
Growth is basically a plant making lots more cells. More cells makes an organism bigger which is growth!
- Intense light allows for efficient photosynthesis - the production of lots of glucose which can be turned into energy via. respiration and used to produce all the new cells for mitosis. So lots of light means lots of photosynthesis and this results in lots of mitosis (new cells).
- Warm temperatures allows for efficient photosynthesis and respiration - meaning lots of energy for mitosis to occur, making lots more cells.
- Water fills plant cells and causes them to elongate - if cells don't have enough water available to them they will stay small!
So keeping that in mind... think about how they relate to the seasons
Spring ❀
What this looks like: Lots of new cells made in spring and they grow in size due to the availability of water! |
Summer ☼
What this looks like: Lots of new cells made in summer but they stay small as there is not enough water available for elongation to occur in the cells! |
Autumn 🍂
What this looks like: The rate of cell division (mitosis) is slow, so growth is slow. |
Winter ⛄
What this looks like: Barely any mitosis happening at all. |
Summer
When looking at growth rings, the rings that appear darker are created in Summer - when lots of cells are made but due to being so small, are tightly packed together and appear darker. Spring When looking at growth rings, the rings that appear lighter are created in Spring, when lots of cells are made and due to the abundance of water, are able to elongate and get bigger in size - next to the smaller cells they appear lighter in colour. |
One more explanation for good measure...
In summer there is adequate light but not adequate water, so cambium cells undergo mitosis however the resulting cells do not get larger/elongate due to the lack of water. This causes a dense area of cells being produced. The cell walls are closer to each other and this causes the cells to appear as a dark ring. In autumn and winter there is little cell division, not many cells are produced. In spring, there is adequate light and water, so cambium cells undergo mitosis at a high rate and there is enough water available to enter the cells and cause them to get larger. This creates the effect of a 'lighter ring' because the cell walls are all spread apart and the density of cells in one area is a lot less.
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