Plants and Water
In this topic you will:
- find out how plants absorb water
- discover how water moves from a plant’s roots to its leaves.
Key words
- root hairs
- xylem vessels
How root hairs absorb water
Plants, like all living things, are made mostly of water. Plants need to absorb water almost all of the time. They do this through their roots. The roots absorb water from the soil.
Soil is made up of tiny particles of rock. The particles are usually irregularly shaped, and all sorts of different sizes. They do not pack together tightly. There are spaces between them, which are filled with air and water.
You may remember that special cells called root hairs grow out of the surface of roots. Root hair cells provide a really big surface through which water and mineral ions can be absorbed into the plant.
This photograph of root hairs was taken with a microscope. Root hairs are very tiny, but you can just see them without using a lens.
This diagram shows a root hair growing in between soil particles. Each root hair is part of a single cell.
Water moves into the root hair cell from the soil. It passes through the cell wall and the cell membrane of the cell, and into the cytoplasm.
Minerals, including magnesium and nitrate, also move into the root hair cell, along with the water. Magnesium and nitrate are in the form of ions (see Topic 2.4), dissolved in the water between the soil particles.
Quick Fact
Root hairs are so small that they can only be seen clearly under a microscope — yet they play a huge role in a plant's survival.
Important Concept
Root hair cells provide a large surface area for water and mineral ion absorption. They are essential for helping plants take in the nutrients they need to survive.
Common Mistake
Some learners confuse root hairs with whole roots. Root hairs are not separate roots — they are extensions of individual root cells that increase surface area for absorption.
Questions
Show Answer
Root hair cells have a long, thin extension that increases the surface area for absorbing water and mineral ions from the soil. Their thin cell walls make it easier for substances to move through.
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Plants need nitrate ions to make proteins for growth and repair. They need magnesium ions to make chlorophyll, which is essential for photosynthesis.
How water moves up the plant
This diagram shows what a root looks like if you cut it across. You can see the root hairs on the outside of the root. In the centre, there are some very special cells called xylem vessels. These are the water transport system of the plant.
After water has been absorbed into a root hair cell, it moves from the outside of the root towards the inside. It goes into the xylem vessels in the centre of the root.
Xylem vessels are long, tube-like cells. They are dead cells – all of their contents, such as cytoplasm and a nucleus, have disappeared. All that is left is their cell walls, with an empty space inside. Their end walls have completely disappeared too. Many xylem vessels stack on top of one another, making long, empty tubes that reach all the way from the roots, up to the highest parts of the plant.
The wood in a tree trunk is made up of xylem vessels. They carry water from the roots all the way to the top of the tree.
This photograph shows some xylem vessels. It has been taken with a microscope. The actual diameter of a xylem vessel is only about 0.05 mm – although, as you can see in the photograph, they come in a range of different sizes.
Quick Fact
Water can rise over 100 metres in tall trees through xylem vessels — using a combination of root pressure, capillary action, and transpiration pull.
Important Concept
Xylem vessels transport water and mineral ions in one direction only — from the roots to the leaves. They are essential for keeping plants hydrated and supplying minerals for photosynthesis and growth.
Common Mistake
Some students think that xylem vessels are alive because they are made of cells. In fact, xylem vessels are dead cells — they have no nucleus or cytoplasm and act as hollow tubes for water transport.
Xylem in roots and stems
These two diagrams show the position of xylem in the roots (left) and stems (right) of a plant. They are cross-sections – you are looking down on a cut surface across a root and across a stem. The xylem vessels are shown in red.
Think Like a Scientist
Investigating transport in a celery stalk
In this experiment, you will try to follow the path of water as it moves up through a stalk.
You will need:
- a stalk of celery, pak choi or another plant that has a thick stalk – it works best if there are leaves at the top of the stalk, a small container, such as beaker or Petri dish, some water mixed with a coloured dye (e.g. blue ink, or methylene blue stain).
Safety
Be sure to put the stalk onto a non-slip surface before you try to cut it.
Cut away from you, so that if the blade slips, it does not hit your hand.
Method
1 Put some of the coloured water into your small container. You need no more than 1 to 2 cm depth.
2 Cut across the lower end of the stalk (not the leaf end), making sure that the cut end is clean and is not blocked or damaged.
3 Stand the stalk in the coloured water. Make sure you stand the cut end in the water.
4 Every five minutes or so, look at the stalk. You may be able to see the blue dye moving up through it. This can sometimes happen very quickly, or sometimes very slowly – so be prepared!
5 When the dye has reached the top of the stalk, take the stalk out of the container. Rinse the blue dye off the end of the stalk, using tap water.
6 Put the stalk down onto a cork board or other non-slip surface. Use a knife to cut across the end of the stalk, about half way up.
You should see something like this:
Questions
Show Answer
Your drawing should show small blue circles or dots at the edge of each vascular bundle. These are the xylem vessels where the dye has travelled.
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The blue spots mark the location of xylem cells, which transport water through the plant.
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This ensures that only the lower end of the stalk is in contact with the dye, so the movement of water can be clearly observed as it rises through the xylem.
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Washing removes any surface dye, so that only the dye that has moved up inside the stalk remains visible — this gives accurate results.
Show Answer
In a complete plant, roots would absorb the water and minerals from the soil and transport them into the xylem. The xylem vessels in the stalk would then carry the water upwards to the leaves.
Planning an experiment
Method
Now that you have done the experiment to see how coloured water moves up a stalk, you are going to think about how you could modify this method to test a hypothesis. After you have had your plan checked, you may be able to do your planned experiment.
Show Answer
Warmer temperatures may cause water to move faster due to increased evaporation and transpiration rates at the top of the stalk.
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If the temperature increases, then the speed of water movement up the stalk will increase.
Show Answer
Use several celery stalks and place them in dye at different temperatures (e.g. fridge, room temperature, warm water bath). Measure how far the dye travels in a fixed time.
Show Answer
You should include: stalks, dye, containers, thermometers, stopwatch; change: temperature; measure: distance dye moves; keep same: stalk size, dye type, time; risk: cutting safety and dye handling.