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Xylem

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Biology

Xylem is a specialised vascular tissue structure that, in addition to transporting water and inorganic ions, will also provide mechanical support to the plant. Together with the phloem, the xylem forms a vascular bundle. To learn about the differences between xylem and phloem, take a look at our article Phloem.

The function of the xylem

The xylem transports water and inorganic ions in a unidirectional flow from the roots (sink) to the leaves (source) in a process known as transpiration. To understand this process, we first need to learn what properties of water allow this to occur.

A source is the plant region where food is made, such as leaves.

A sink is where food is stored or used, such as the root.

Water properties

Water has three properties that are extremely important to maintain the transpiration stream up the plant. These properties are adhesion, cohesion and surface tension.

Adhesion

Adhesion refers to the attraction between two different substances. In this case, the water molecules are attracted to the walls of the xylem. Water molecules will cling to the xylem walls because xylem walls are charged.

The water molecules move via capillary action. This creates greater tension within the xylem walls, allowing for efficient water movement.

Capillary action describes the movement of liquids up a hollow space due to cohesion, adhesion and surface tension.

Cohesion

Cohesion refers to a molecule’s ability to stick together with other molecules of the same kind. The cohesive forces in water are created through hydrogen bonds. Hydrogen bonds form between water molecules because water is polar (it has an imbalanced charge distribution).

Polar molecules come about due to the unequal sharing of electrons. In water, the oxygen atom is slightly negative, and the hydrogen atom is somewhat positive.

Surface tension

In addition to cohesion and adhesion, the surface tension of the xylem sap (water with dissolved minerals) is also significant. A substance having surface tension means that it will tend to occupy the least space possible; cohesion allows this to happen, as it lets molecules of the same substance stay close together.

The surface tension of the xylem sap is created by the transpiration stream, which moves the water up the xylem. The water is pulled towards the stomata, where it will evaporate.

Adaptations and structure of xylem cells

The xylem contains four types of cells:

  • Tracheids - long and narrow hardened cells with pits.
  • Xylem vessel elements - meta-xylem (the primary part of xylem that differentiated after proto-xylem) and proto-xylem (formed from primary xylem and matures before the plant organs completely elongate)
  • Parenchyma - xylem’s only living tissue, thought to play a part in the storage of starch and oils.
  • Sclerenchyma - xylem fibres

Tracheids and xylem vessel elements will conduct the transport of water and minerals. Xylem possesses several adaptations that allow for efficient water transport:

  • No end walls between the cells - water can flow using mass flow. Cohesion and adhesion (properties of water) play a crucial role here as they cling to each other and the walls of the xylem.
  • Cells are not living - in mature xylem, the cells are dead (except for the parenchyma storage cells). They do not interfere with the mass flow of water.
  • One-way flow system allows for the continuous upward movement of water driven by the transpiration stream.
  • Narrow vessels - this assists the capillary action of the water and prevents breaks in the water chain.

Mass flow describes the movement of fluid down a pressure gradient.

Xylem in plant support

Lignin is the primary supportive element of the xylem tissue. The main two features are:

  • Lignified cells - lignin coats the walls of the xylem for extra strength, helping to withstand the water pressure in the plant and preventing the plant’s collapse.
  • Walls possess pits - pits form where lignin is thinner. These allow the xylem to withstand the water pressure as it fluctuates throughout the plant.

Pits in the xylem walls are a feature of secondary growth. They are not perforations!

Vascular bundle arrangement in monocots and dicots

There are differences in the distribution of the vascular bundles in monocotyledonous (monocot) and dicotyledonous (dicot) plants. In short, the vascular bundles containing xylem and phloem are scattered in monocots and are arranged in a ring-like structure in dicots.

First, let’s cover the main differences between monocots and dicots.

What is the difference between monocots and dicots?

There are five main features that are different between monocots and dicots:

  1. The seed: monocots will possess two cotyledons, while dicots will only have one. A cotyledon is a seed leaf that resides within the seed embryo to supply nutrition to the embryo.
  2. The root: monocots have fibrous, thin branching roots growing from the stem (e.g. wheat and grasses). Dicots have a dominant central root from which smaller branches will form (e.g. carrots and beetroots).
  3. Vascular structure of the stem: the bundles of xylem and phloem are scattered in monocots and are arranged in a ring-like structure in dicots.
  4. Leaves: monocot leaves are narrow and slender, usually longer than dicot leaves. Monocots will also have parallel veins. Dicot leaves are smaller and broader; they will exhibit isobilateral symmetry (opposite leaf sides are similar). Dicots will have net-like leaf veins.
  5. Flowers: monocot flowers will be in multiples of three, while dicot flowers will have multiples of four or five.

The isobilateral symmetry of leaves describes how opposite leaf sides are the same.

Vascular bundle arrangement in the plant stem

In the stems of monocots, the vascular bundles are scattered throughout the ground tissue (all tissue that is not vascular or dermal). The xylem is found on the inner surface in the bundle, and the phloem is on the outer. Cambium (an actively dividing layer of cells that promotes growth) is not present.

Cambium is a layer of unspecialised cells actively dividing for plant growth.

In the stems of dicots, the vascular bundles are arranged in a ring-like structure around a cambium. Xylem is present in the cambium ring’s inner part, and phloem is present at the exterior. Sclerenchyma tissue comprises thin and narrow non-living cells (when mature). Sclerenchyma tissue does not have any internal space, but it plays an essential role in plant support.

Vascular bundle arrangement in the plant root

Monocots have a fibrous root, and dicots have a tap root.

When you look at the cross-section of the root, in general, a single ring of xylem will be present in monocots. Xylem is surrounded by phloem, which is different from their monocot stems. The monocot root has more vascular bundles than the dicot root.

In the dicot root, the xylem is in the middle (in an x-shaped manner), and the phloem is present in clusters around it. Cambium separates xylem and phloem from each other.

Xylem - Key takeaways

  • Xylem is a specialised vascular tissue structure that, in addition to transporting water and inorganic ions, will also provide mechanical support to the plant. Together with phloem, they form a vascular bundle.
  • Xylem is adapted to transport the sap, having no end walls, one-way flow system, non-living cells and narrow vessels. In addition to the xylem’s adaptation for transport, water possesses adhesion and cohesion to maintain the water flow.
  • Lignin lines the walls of the xylem to provide mechanical strength to the plant.
  • Xylem distribution in monocots and dicots varies. In the stem of dicots, the xylem is arranged in a ring formation and in monocots, the xylem is scattered throughout. In the root of dicots, xylem is present in an x-shape which phloem around it; in monocots, xylem is present in a ring formation.

Xylem

Water and dissolved inorganic ions.

Xylem is a specialised vascular tissue structure that, in addition to transporting water and inorganic ions, will also provide mechanical support to the plant.

To transport water and inorganic ions and provide mechanical support to the plant.

Examples of the adaptations:


  1. Lignified walls with pits to withstand fluctuating water pressures and provide support to the plant. 
  2. No end walls between the non-living cells - water can mass flow without being stopped by the cell walls or contents of the cells (that would be present if cells were living).
  3. Narrow vessels - supports capillary action of the water.

Lignin.

Final Xylem Quiz

Question

Plants have two transport systems. Xylem transports water and inorganic ions. What is the name of the other system and what does it transport?

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Answer

Phloem. It transports amino acids and sugars.

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Question

Xylem and phloem transport substances in a bi-directional flow. True or False?


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Answer

Partially true. Xylem transports water in one direction and phloem transports materials bi-directionally.

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Question

What are the two main properties of water molecules that help the mass flow of water?


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Answer

Adhesion and cohesion.

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Question

How does cohesion occur between the water molecules?


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Answer

The cohesive forces between water molecules occur due to hydrogen bonds. Water molecules are charged due to an unequal sharing of electrons. This allows for hydrogen bonds between water molecules.

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Question

Fill in the blanks about adhesion. 


Adhesion is the _________ between the two __________ substances. In xylem, water molecules are _______ to the ________ of xylem. Water molecules are charged and xylem ________ are too. Water will move via ________ action up the plant.

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Answer

Adhesion is the attraction between the two different substances. In xylem, water molecules are attracted to the walls of xylem. Water molecules are charged and xylem walls are too. Water will move via capillary action up the plant.

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Question

What will create surface tension in xylem’s sap?

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Answer

Transpiration stream.

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Question

What are the four types of cells present in xylem?


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Answer

Tracheids, xylem vessel elements, parenchyma and sclerenchyma.

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Question

What would happen if xylem had end walls?


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Answer

Mass water flow would be disrupted.

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Question

How are the pits in the lignified xylem walls useful?


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Answer

Pits allow xylem to withstand the water pressure as it fluctuates throughout the plant.

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Question

How is the xylem and phloem complex called?


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Answer

A vascular bundle.

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Question

What is cambium?


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Answer

An actively dividing layer of cells that promotes growth.

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Question

What do the primary and secondary growths do?


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Answer

The primary growth increases the length in shoots and the root. The secondary growth causes an expansion in plant’s size.

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Question

How will the vascular bundles be arranged in the dicot plant’s stem?


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Answer

In a ring-like manner.

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Question

How are xylem and phloem arranged in a dicot root?

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Answer

Xylem appears in an x-shaped pattern and phloem is arranged around it.

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Question

Name two differences between the structure of xylem and phloem.


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Answer

The two differences (any differences are correct):


  1. No end walls in the xylem but present in the phloem.
  2. One way system in xylem but bi-lateral movement in phloem.

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