Xylem Functions, Definition and Structure in Plants

What is Xylem?

The xylem of plants is a tissue that transports water and nutrients from the root of the plant to other parts of the plant such as leaves and stems. It is found in vascular plants. This tissue and phloem are the two transport tissues. In plants, there are different plant tissues which include the reproductive tissue, meristematic tissues, and permanent tissues. The complex permanent tissues are the vascular tissues which include the xylem and phloem. They are present in all vascular plants including angiosperms, ferns, horsetails, seedless club mosses, and gymnosperms.

Wood showing its annual rings (secondary xylem)
Wood is the best-known xylem tissue

Phloem tissues transport sugars from the leaves of the plants (where photosynthesis takes place) to the rest of the body whereas, the plant xylem tissues are actively involved in the transportation of minerals and water from the root to the leaves and shoots. This vascular tissue gives support to other tissues that are soft in vascular plants and it is what distinguishes a vascular plant from a nonvascular plant.

It is made up of several specialized water-conducting cells called tracheary elements. Also, this tissue has fiber cells that render support and possesses parenchyma, which are thin-walled unspecialized cells that store various substances. The tracheary elements of the plant xylem consist of cells called vessels members and tracheids. Tracheids and vessels are usually narrow, hollow, and elongated. The vessel members, however, are more specialized than the tracheids which are the only water-conducting cell types in most seedless vascular plants and gymnosperms.

There is a thin modified primary cell wall called the pit membrane that water passes through from the tracheid to another tracheid. The pit membrane helps to prevent damaging air bubbles from passing. Vessel members, on the other hand, are the main water-conducting cells in angiosperms. They are distinct for the areas known as perforations that lack primary and secondary cell walls. It is through these perforations that water flows from vessel to vessel without any hindrance. However, fractures and disruptions from air bubbles are likely to occur.

Definition in biology

The xylem can be defined in biology as a specialized tissue in vascular plants that carry out the transportation of dissolved nutrients and water from the root to the leaves and stem of the plant. It gives support and storage to the plant. The xylem of plants is made up of complex systems and many cell types involved in the transportation of the dissolved minerals and water in order to support the plant and as well give nutrition. These tissues are found in the leaves and stems of vascular plants.

The formation of the plant xylem starts when the dividing cells of the growing root and shoot tips produce the primary xylems. Then, as the plant expands in its circumference, the secondary xylem is formed, thereby, building a ring of new tissues around the original primary xylem tissues. The primary xylem cell eventually dies losing its conducting function and thus forming a hard skeleton that only supports the plant.

Therefore, the secondary xylems in woody plants are the major part of a mature root or stem. Only the outer secondary xylem (sapwood) in the trunk and older branches of a large tree does the water conduction whereas, the inner part (heartwood) consists of dead but structurally strong primary xylems. Therefore, the age of a tree in cold or temperate climates can be determined by basically just counting the number of annual rings that are formed at the base of the trunk.

What does the xylem do?

The xylems of plants specialize in the transportation of water and dissolved nutrients from the roots of the plant to the stems and leaves. It basically moves water and nutrients to other parts of the plants. The cohesive forces between water molecules function as a connecting process for the conduction of water within the vascular system. These vascular tissues in plants are seen to be essential and an added advantage as vascular plants that possess xylems tend to grow higher than nonvascular plants.

These tissues in vascular plants function in transporting water which is a crucial element for plant growth and also provides support to the plant due to its rigid form. The xylem in a plant also conveys phytohormonal signals. This plant tissue conveying water across the plant can be explained by the Cohesion-Adhesion theory. This theory is a hypothesis that attempts to explain how water goes against gravity by moving upwards across the plant.

Transpiration in plants is the main factor responsible for water moving up in order to replace the water that has been lost due to evaporation. The vascular tissue then takes water from the roots and transports it to other parts of the plants. As mentioned earlier, there are several cells that are involved in this water conduction and transportation process. The tracheary elements act passively for water transportation and this water makes it to the stem and leaves from the roots based on two main factors. These factors include root pressure and transpirational pull.

The root pressure takes place as a result of osmosis which allows water from the soil to get to the roots of the plants. Transpirational pull occurs as a result of water loss through the process of transpiration from the leaves. Then, the surface tension pulls the water upwards within the xylems. The transport mode is passive transport and there is water loss via transpiration that leads to high surface tension. This tension, in turn, leads to negative pressure in the xylem. Subsequently, from the roots, water is moved towards the apical parts of the plants which are several meters high away from the ground.

Types

  • Primary xylem
  • Secondary xylem
the primary and secondary xylem are the two types of xylem
Photo credit: http://www.bio.miami.edu/

This plant tissue is divided into two types based on its structure, function, development, and role. However, the two types of xylem which include primary and secondary xylem carry out the same function, and their type of growth for formation is what categorizes them.

Primary xylem

This tissue type enables the plant to grow tall and makes the roots of the plant longer. It is at the tips of the stems, flower buds, and roots that the primary xylem occurs during the primary growth of the plant from the procambium. Primary growth involves the protoxylem and metaxylem. After the protoxylem, the metaxylem develops, and this happens before the secondary xylem develops. The metaxylem possesses wider tracheids and vessels than the protoxylem.

Secondary xylem

This tissue type is formed during the secondary growth of the plant from vascular cambium. The secondary xylem helps the plant to grow wider with time. This happens after each yearly growth. Thus, the age of trees can be determined because the secondary xylems give dark rings to the plant. The wide tree trunks are typical examples of the secondary growth of plants.

This tissue is found in all conifers and angiosperms. The secondary xylem of the majority of conifers which become tall trees is used and sold as softwood. Amongst the angiosperms, this plant tissue type is rare in monocots. The majority of the angiosperms that are non-monocot are used and sold as hardwood. Furthermore, the secondary xylem is also seen in Gnetophyta and Gnikgophyta which are members of the gymnosperm groups. Also, to a lesser extent, it is found in members of the Cycadophyta.

Structure

The xylem structure can be seen in vascular bundles found in non-woody parts of woody plants and non-woody plants. In woody plants, the secondary xylem structure is laid down by a meristem called the vascular cambium. Moreso, the structure in many ferns is a stellar arrangement that is not divided into bundles. Usually, the branching pattern exhibited by the xylem structure follows Murray’s law.

The structure is made of distinctive cells which are the long tracheary elements that transport water. Tracheary elements which include the vessel elements and tracheids are differentiated by their shapes. The vessel elements are shorter and connect together into long tubes referred to as vessels. Parenchyma and fibers are the other two types of cells contained in the xylem. Therefore, the vascular tissue structure contains four types of cells.

Types of xylem cells

  • Tracheids
  • Xylem vessels
  • Xylem fiber
  • Xylem parenchyma

Tracheids

The tracheids structure is tube-like with a tapering end. These cells are dead and have a thick lignified cell wall. They lack protoplasm and are seen in angiosperms and gymnosperms. Tracheids function in the transportation of water and minerals.

Vessels

The structure of the vessels is long and cylindrical which makes them appear tube-like. They are found in angiosperms. Their cell walls are lignified and have a large central cavity. These vessels lose their protoplasm and are therefore dead at maturity. The xylem vessels have many cells called vessel members. Through a perforation in the common walls, these vessel members are interconnected. Hence, they carry out the conduction of water, minerals and also give the plant mechanical strength.

Fibers

These dead cells provide mechanical support to the plant and are involved in water transportation. They contain a central lumen and lignified walls.

Parenchyma

These cells store food material and are said to be the living xylem cell. They play a role in the storage of fats, and carbohydrates. Also, they assist in the reduced distance water transportation and aid water conduction. The parenchyma has a prominent nucleus and protoplast with a cell wall that is usually cellulosic and thin.

Primary and secondary xylems both have living parenchyma cells that are colorless and have large vacuoles. The parenchyma has chloroplasts that are found in woody plants, angiosperms, and herbaceous plants. Through the parenchymatous cell, the parenchyma conducts water in an upward direction.

Food nutrients are stored in the parenchyma in the form of tannins, crystals, fats, and starch. Parenchyma cells are connected to vessels or tracheids through an outgrowth called tyloses. Furthermore, parenchyma maintains cavitation or embolism which aids in continuing the functions of vessels and tracheids.

Xylem Functions

  • The main function of the xylem is to support and strengthen the organs and tissues of a plant.
  • Xylem function in maintaining the structure of the plant and prevents the plants from bending.
  • The transportation of water, soluble organic ions, and sap.
  • The cells form long tubes that function in the movement of sap which is the mixture of water and nutrients.
  • It gives additional support to the plant as it allows water and minerals to move to the plant’s organs at higher regions.
  • Water moves up through the plant xylem by sticking to the xylem cell. However, as a plant grows tall, the xylem function to set an upper limit on the growth of tall trees.

Xylem of plants

Angiosperms and other vascular plants

The flowering plants (angiosperms), gymnosperms (naked seed-producing plants), and pteridophytes are groups of vascular plants. These groups of plants, however, can be differentiated based on their xylem tissues.

The xylem tissues of the angiosperm have vessels that are not in the tissues of ferns or gymnosperms. Rather, the tissues of gymnosperms or fern possess only tracheids instead of vessels. The vessels are the main conducive element in the majority of angiosperms. Hence, the xylem of plants that are angiospermous has vessels whereas, the xylem of plants that are gymnospermous have only tracheids.

Nevertheless, the vessels and tracheids at maturity actually lose their protoplast and become non-living and hollow. The polymer lignin deposits and forms a secondary cell wall. The secondary walls of the xylem vessels are usually thinner than the tracheids. They both form pits on their lateral walls. Moreso, the vessels conduct more water than the tracheid.

Monocot and Dicot

Angiosperms are grouped into monocots and eudicots. These two groups are actually grouped according to the number of cotyledons they possess. Monocots have one cotyledon whereas, dicots possess two cotyledons. Examples of monocots are bananas, palm trees, grasses, bamboos, orchids, grasses, and palm trees while examples of eudicots include maples, oaks, strawberries, sunflowers, roses, magnolias, and sycamores.

Dicots and monocots can be differentiated too according to their xylem tissues. The xylem of the dicot root appears star-like and the phloem is in between the prongs of the xylem. Monocots, on the other hand, possess alternating xylem and phloem tissues. The xylem vessels are another factor that marks the difference. The vessels in the roots of dicots are angular or polygonal. Monocot roots, on the other hand, have vessels that are rounded or oval. In the roots of dicots, the xylem-phloem elements are fewer than in the roots of monocots.

Xylem in a monocot plant stem vs Xylem in a Dicot plant stem
A diagram illustrating the arrangement of vascular bundles in a monocot and dicot
Photo Credit: http://www.bio.miami.edu/

Dicots and monocots also have differences in their stems. In the monocot stem, the vascular bundles which comprise xylem tissues, phloem, and vascular cambium are scattered. The stem of dicots, on the other hand, has their vascular bundles arranged in a ring pattern. Dicots have secondary growth and form growth rings (annual rings) in their stems.

Woody plants

Dicots are subdivided into herbaceous dicots and woody dicots. There are two types of xylem in woody plants- primary and secondary. The primary xylem in a plant is responsible for the increase in length of the plant and the primary growth. Whereas, the secondary xylems (wood) in a plant are responsible for the increase in girth of the plant and secondary growth. The age of trees can be determined because the secondary xylems give dark rings to the plant.

Secondary xylem in a woody plant
A diagram showing the Secondary xylem in a woody plant
Photo Credit: http://pollen.utulsa.edu/

The group of plants that produces wood is the angiosperms and gymnosperms. The wood of the angiosperm is referred to as hardwood while the wood of the gymnosperm is referred to as softwood. Hardwood is more compact and dense compared to softwood.

Xylem vs Phloem

Phloem and xylem are the two main types of vascular tissues that carry out the transportation process in plants. They create a vascular bundle that works together as a unit. However, there are differences between them. The contrast of these tissues can be seen in their movement and function. The movement of the xylem in a plant is unidirectional whereas, the movement of the phloem in a plant is bidirectional.

In vascular plants, xylem transport water and provide support to the plant. Its presence in a vascular plant is the reason why they grow higher than nonvascular plants. Phloem in vascular plants, transport nutrients such as sugar, organic molecules, and proteins which enable the plant to stay alive and also reproduce. Hence, the main function of the xylem is to transport water and minerals from the roots to other parts of the plants whereas, the phloem transport nutrients and food from the leaves of the plant to other parts of the plant.

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