If you're curious about how plants move water and nutrients, you'll want to know about xylem. It's a special tissue that not only carries water and minerals, but also helps support the plant. The xylem is part of a larger system, along with the phloem, called the vascular bundle. If you want to understand the contrast between xylem and phloem, check out our post on Phloem.

The function of the xylem

Xylem is responsible for carrying water and inorganic ions in a one-way flow from the roots (where food is stored or used) to the leaves (where food is made) in a process called transpiration. To appreciate how this works, it's important to understand the unique properties of water that enable this process to take place. A source, such as leaves, is where food is produced, while a sink, such as the root, is where food is used or stored.

Water properties

Water has three crucial properties that enable it to move up the plant via the transpiration stream. These properties are adhesion, cohesion and surface tension. Adhesion is the attraction between different substances, and in the case of xylem, the water molecules are attracted to its walls. The xylem walls are charged, which causes the water molecules to cling to them and move via capillary action. Capillary action, created by cohesion, adhesion, and surface tension, describes the movement of liquids up a hollow space. Cohesion refers to the ability of molecules to stick together, and in water, this is due to hydrogen bonds. The surface tension of the xylem sap is also significant because it allows the water to occupy the least space possible. The transpiration stream creates this surface tension, which moves the water up the xylem and towards the stomata, where it evaporates.

Adaptations and structure of xylem cells

The xylem is composed of four types of cells: tracheids, xylem vessel elements, parenchyma, and sclerenchyma. Tracheids and xylem vessel elements are responsible for conducting the transport of water and minerals. Xylem has several adaptations that enable efficient water transport. For instance, there are no end walls between the cells, which allows water to flow using mass flow. Cohesion and adhesion play a crucial role in this process because they cause water molecules to cling to each other and the xylem walls. Additionally, mature xylem cells are dead (except for the parenchyma storage cells), which does not interfere with the mass flow of water. A one-way flow system allows for the continuous upward movement of water driven by the transpiration stream. Moreover, narrow vessels assist the capillary action of the water and prevent breaks in the water chain. Mass flow refers to the movement of fluid down a pressure gradient.

Xylem in plant support

Lignin is the primary supportive element of the xylem tissue. It has two main features that make it essential for the plant's survival. Firstly, lignified cells refer to the walls of the xylem being coated with lignin for extra strength. This helps to withstand the water pressure in the plant and prevent its collapse. Secondly, the walls of the xylem possess pits where the lignin is thinner. These pits allow the xylem to withstand the fluctuating water pressure throughout the plant. It is important to note that pits in the xylem walls are a feature of secondary growth and 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?

Monocots and dicots differ in five main features. Firstly, the seed of monocots will have two cotyledons, whereas dicots will only have one. Cotyledons are seed leaves that supply nutrition to the embryo. Secondly, monocots have fibrous, thin branching roots growing from the stem, while dicots have a dominant central root from which smaller branches will form. Thirdly, the vascular structure of the stem differs between monocots and dicots. Monocots have scattered bundles of xylem and phloem, while dicots have a ring-like structure. Fourthly, monocot leaves are narrow and slender, usually longer than dicot leaves. Monocots also have parallel veins, while dicots have net-like leaf veins. Lastly, monocot flowers will be in multiples of three, while dicot flowers have multiples of four or five. Additionally, dicot leaves exhibit isobilateral symmetry, which means opposite leaf sides are similar.

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. In contrast, 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, while dicots have a tap root. In general, a single ring of xylem is present in monocots when you look at the cross-section of the root. The xylem is surrounded by phloem in monocot roots, which is different from their stems. The monocot root has more vascular bundles than the dicot root.

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

Xylem is a specialised vascular tissue that transports water, inorganic ions and provides mechanical support to the plant. Together with phloem, they form a vascular bundle. Xylem is adapted to transport sap, having no end walls, a one-way flow system, non-living cells, and narrow vessels. The lignin lines the walls of the xylem to provide mechanical strength to the plant. Xylem distribution varies in monocots and dicots. In the stem of dicots, the xylem is arranged in a ring formation, while in monocots, it is scattered throughout. In the root of dicots, xylem is present in an x-shape with phloem around it, while in monocots, it is present in a ring formation.

Xylem

What does xylem transport?

Water and dissolved inorganic ions.

What is xylem?

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

What is the function of xylem?

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

How are xylem cells adapted to their function?

Examples of the adaptations:Lignified walls with pits to withstand fluctuating water pressures and provide support to the plant. 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).Narrow vessels - supports capillary action of the water.

What substance strengthens xylem?

Lignin.

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