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".
Xylem Function
Let's start by looking at the function of xylem cells.
Plant xylem delivers water and nutrients from the plant-soil interface to stems and leaves, and provides mechanical support and storage as well. 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.
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.
To understand this process, we first need to learn what properties of water allow this to occur.
Water properties
Water has three properties that are essential for maintaining 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.
Fig. 1 - The cohesive and adhesive properties of water
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.
Fig. 2 - The transpiration stream in xylem
Adaptations and Structure of Xylem Cells
Xylem cells are adapted to their function. By losing their end walls, the xylem forms a continuous, hollow tube, strengthened by a substance called lignin.
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.
Fig. 3 - The structure of xylem
Xylem in plant support
Lignin is the primary supportive element of the xylem tissue. The main two features are:
- Lignified cells - lignin is a substance that strengthens the cells walls of xylem cells, allowing the xylem to withstand water pressure changes as water moves through the plant.
- 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:
- 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.
- 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).
- 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.
- 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.
- 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.
Fig. 4 - A summary table of the features in monocots and dicots
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.
Fig. 5 - A cross-section of the stem of a dicot and monocot plant
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.
Fig. 6 - A cross-section of root tissue of a dicot and monocot
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.
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