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Xylem

Xylem definition

Xylem
n., plural: xylems
[ˈzaɪ.ləm]
Definition: A type of vascular tissue in plants

Xylem Definition

Xylem is defined as a plant tissue that transfers water and nutrients from roots to all over the plant body, such as stems and leaves. The presence of xylem tissue is one of the distinguishing features separating vascular plants from nonvascular plants. The xylem provides support to other soft tissues present in vascular plants. In 1858 the Carl Negali introduced the term xylem. The term xylem is derived from the Greek xylon (meaning “wood”). Wood is a popular example of xylem.

What is xylem? According to biologists, the xylem is a specialized tissue present in vascular plants for transporting water and dissolved nutrients from roots to the plants’ leaves and stems. It also provides storage and support to the plant (Myburg. A. et al., 2013). In simple terms, xylem is a type of vascular tissue responsible for conducting water throughout the plant body. Xylem comprises complex systems and several types of cells for transporting water and dissolved minerals to support and provide nutrition to plants.

Biology definition: Xylem is a  type of vascular tissue in plants. It is primarily involved in transporting water and minerals (from the roots to the shoot and leaves) and providing structural support. It is found in the stems and leaves of vascular plants. Etymology: Greek “xylon”, meaning “wood”. Compare: phloem.

Xylem vs. Phloem

What are xylem and phloem? Xylem and phloem are vascular tissues responsible for transporting water and food, respectively. How is xylem different from phloem? You can also look at the table below. Also, you may read this for phloem definition and more information.

Table 1: Differences between Phloem and Xylem
Phloem Xylem
Phloem transports nutrients (proteins, glucose, and other organic molecules). Xylem transports water and dissolved minerals.
Takes food synthesized from leaves to transport to other parts of the plant Conduct water from roots to other parts of the plant
The food is transported in both upward and downward directions. The conduction or transportation of water only occurs in an upward direction.
Adenosine triphosphate (ATP–a form of energy) is required for the conduction of food in the phloem Xylem conducts water through transpiration pull (a physical force that pulls water from roots).
Phloem tissues have walls (made up of thin sieve tubes) and are elongated with tubular-shaped structure. Xylem tissues do not have cross walls and have tubular or star-shaped structure.
Present near the periphery of the vascular bundle and have larger fibers. Xylem is present in the middle of the vascular bundle and has smaller fibers.

 

xylem and phloem components
Figure 1: Xylem and Phloem Components. Credit: Kelvinsong – xylem and phloem (diagram), CC BY-SA 3.0

Role of xylem in vascular plants

What is the role of xylem in a vascular plant? The vascular plants grow higher than nonvascular plants due to the presence of xylem tissues that provide support (due to its rigid form) and transports water (a necessary component for plants’ growth) to the various parts of the plant.

Role of Phloem in vascular plants

The phloem of vascular plants is responsible for transporting nutrients including, sugar, proteins, and organic molecules that help plants to remain alive and reproduce.

 

water movement between vascular tissues
Figure 2: Water movement between xylem and phloem tissues. Credit: CNX OpenStax – (photo), CC BY-SA 4.0.

 

In plants, the different types of tissues include the meristematic tissues, the permanent tissues, and the reproductive tissues. The permanent tissues are further classified into fundamental tissues and complex permanent tissues. The complex permanent tissues include the vascular tissues, particularly, xylem and phloem.

Xylems of Angiosperms and Other Vascular Plants

Angiosperms (known as flowering plants) are one of the major groups of vascular plants. The others are gymnosperms (naked seed-producing plants) and pteridophytes (e.g. ferns). These groups can be distinguished based on their xylem tissues. For instance, the xylem tissues of flowering plants contain xylem vessels that are absent in the xylem tissues of gymnosperms or ferns. They have no xylem vessels but only tracheids. In most angiosperms, the xylem vessels serve as the major conductive element.

Nonetheless, both tracheids and xylem vessels lose their protoplast at maturity and become hollow and non-living. The polymer lignin is deposited forming a secondary cell wall. The xylem vessels, though, have thinner secondary walls than the tracheids. Then, both of them form pits on their lateral walls.

The xylem vessel is a series of cells called vessel members (or vessel elements), each with a common end wall that is partially or wholly dissolved. This is in contrast to a tracheid, which is an individual cell. Also, the tracheid cell is typically longer than the vessel member. However, the vessel member is wider in diameter. Because of this, the xylem vessel conducts more water than the tracheid.

xylem vessel and tracheids of angiosperm
Figure 3: Xylem vessel and tracheids in angiosperm. Source: Modified by Maria Victoria Gonzaga, BiologyOnline.com, from the works of Kelvinsong, CC BY-SA 3.0.

Xylem: Monocot vs Dicot

Angiosperms may be grouped into two major groups: (1) the monocots (e.g. orchids, bananas, bamboos, palm trees, grasses, etc.) and (2) the eudicots (e.g. roses, magnolias, strawberries, sunflowers,  oaks, maples, sycamores, etc.). The two groups are differentiated basically by the number of cotyledons they have — monocots have one cotyledon whereas dicots have two. Apart from the cotyledons, they can also be differed by their xylem tissues.

In particular, the xylem of a dicot root has a star-like appearance (3 or 4-pronged). In between the “prongs” of xylem are the phloem. See Figure 4.  In contrast, the monocot root has alternating xylem and phloem tissues. Another marked difference between the two in terms of xylem tissues is the xylem vessels. Dicot roots have polygonal or angular xylem vessels whereas monocot roots have oval or rounded. The xylem-phloem elements are fewer in dicot roots (typically 2 to 6) than in monocot roots (typically 8 or more).

 

Dicot vs monocot roots
Figure 4: Dicot root vs Monocot root. Credit: CNX OpenStax – (photo), CC BY 4.0

Apart from the roots, the dicots and the monocots have apparent differences in their stems. The vascular bundles (i.e. a vascular bundle consists of phloem and xylem tissues, plus vascular cambium) of a monocot stem are scattered whereas in dicot stems they are arranged in a ring pattern. Furthermore, dicots have secondary growth. In their stems, they form growth rings (annual rings). Thus, this leads to a subgroup of dicots: herbaceous dicots (e.g. sunflower stems) and woody dicots (e.g. tree stems with woods).

 

Dicot vs monocot stems
Figure 5: Dicot stem vs Monocot stem. Credit: CNX OpenStax – (diagram), CC BY 4.0.

In woody plants, there produce two types of xylems: (1) primary xylem and (2) secondary xylem. The primary xylem is responsible for the primary growth or the increase in length. The secondary xylem (also called wood) is for secondary growth, which is the increase in girth.

Angiosperms are not the only ones that produce wood (secondary xylem), though. Gymnosperms also produce wood.  The angiosperm wood is called hardwood whereas gymnosperm wood is called softwood. The name is due to hardwood being more compact and denser than softwood. If you will recall, the angiosperms have xylem vessels apart from tracheids. Most gymnosperms have only tracheids. Thus, this makes many hardwoods denser than softwoods. However, there are exceptions. Yews and longleaf pines are softwoods that are extremely durable and harder than many other hardwoods.

hard wood and soft wood
Figure 6: SEM images of hardwood (top) vs softwood (bottom). Notice the pores present in the hardwood but not in the softwood. Credit: Mckdandy – SEM images of Oak (top) and Pine (bottom), CC BY-SA 3.0.

Types of Xylem

On the basis of structure, development, function, and role of xylem tissue, the biologists divided xylem divided into two main types, i.e., primary and secondary. These two types of xylem perform the same function and are categorized by the type of growth for their formation.

Primary xylem

The primary growth of plant formation of primary xylem occurs at the tips of stems, roots, and flower buds. Also, the primary xylem helps the plant to grow taller and makes the roots longer. Thus, it occurs first in the growing season, so this is called primary growth. The purpose of primary and secondary xylem is to transport water and nutrients.

Secondary Xylem

With the secondary growth of the plant, secondary xylem is formed that helps the plant to get wider over time. An example of the secondary growth of plants is wide tree trunks. It happens each year after the growth. Plus, the secondary xylem gives dark rings that determine the age of trees.

Structure of Xylem

Xylem consists of four types of elements: (1) xylem vessels, (2) tracheids, (3) xylem fiber, and (4) xylem parenchyma.

Xylem vessels

The xylem vessels are present in the angiosperms. They have a long cylindrical structure and have a tube-like appearance. Walls contain a large central cavity, and walls are lignified. They lose their protoplasm, and thus, are dead, at maturity. They contain many cells (vessel members) that are interconnected through a perforation in common walls. They are involved in the conduction of water, minerals and give mechanical strength to the plant.

Tracheids

These are dead and are tube-like cells with a tapering end. They are found in the gymnosperm and angiosperm. These cells have a thick lignified cell wall and lack protoplasm. The main function they perform is water and mineral transportation.

Structural components of xylem tissue
Figure 7: Structural components of xylem tissue. Credit: QS Study.

Xylem fibers

These are dead cells containing central lumen and lignified walls; they provide mechanical support to the plant and are responsible for water transportation.

Xylem parenchyma

The cells of xylem called parenchyma cells store food material and are considered the living cells of xylem. Moreover, they assist in the reduced distance transportation of water. Also, they are involved in the storage of carbohydrates, fats, and water conduction.

The main characteristics of xylem parenchyma are as follows:

  • The living cells of xylem
  • The cell wall is always cellulosic and thin.
  • Contains prominent nucleus and protoplast
  • The cells are colorless, and they have large vacuoles.
  • Both primary and secondary xylem contains living parenchyma cells.
  • The components of parenchyma cells such as fats and proteins vary seasonally.
  • They may be subdivided by septa, and they compose of crystal containing parenchyma cells that have lignified walls.
  • Xylem parenchyma also consists of chloroplasts that are present in angiosperms, woody plants, and herbaceous plants.
  • The vessels form outgrowths called “tyloses ‘are beside both axial and ray parenchyma cells.
  • The parenchyma cells are termed as “contact cells,” which give rise to tyloses.
  • The nucleus and cytoplasm of xylem parenchyma cells migrate into tyloses.
  • Tyloses may develop store a variety of substances.
  • Tylose might differentiate into sclereids.

The major functions of xylem parenchyma are as follows:

  • Xylem parenchyma conducts water in an upward direction through the parenchymatous cell.
  • Stores food nutrients in the form of fats, tannins, crystals, and starch.
  • Through the outgrowth called tyloses connects parenchyma cells of xylem to vessels or tracheids.
  • During a drought or infection, the vascular tissues are being protected by tyloses.
  • Parenchyma cells of xylem are involved in the maintenance incapacity of xylem transport.
  • Cavitation or embolism, meaning the xylem cavity blockage is maintained by parenchyma xylem that helps in continuing the functions of tracheids and vessels.

Characteristics of Xylem Tissue

The xylem structure can be understood by the types or divisions of xylem cells, including fiber cells, parenchyma cells, and tracheary elements.

  • Parenchyma cells are long fibers and formed the soft parts of the plant body.
  • These parenchyma cells provide support to the xylem cells.
  • Tracheary elements are dead cells that become hollow strands to let water and minerals flow through them.
  • Both vessels and tracheids (tracheary elements) are hollow, elongated, and narrow. However, the vessels are more specialized than tracheids to help flow the xylem sap.
  • Vessels also contain perforation plates that help in connecting different vessel elements into one continuous sheet of vessels.
  • Xylem also contains several forms of thickenings, which are found in different patterns, rings, and others to maximize the structural support of the plants.
  • The xylem appears as star-shaped when observed under the microscope.

Xylem Function

Xylem transports water and dissolved minerals as well as provides mechanical support to the plant. They also convey phytohormonal signals in the plant body. Cohesive forces between water molecules work as a connecting way for the conduction of water within the xylem vascular system. Below are the precise functions of the xylem.

  • Support: Xylem provides support and strength to the parts of a plant, including tissues and organs, to maintain the plant’s structure and prevent plants from bending.
  • Xylem sap: Xylem vascular system consists of long tubes that allow the flow of water, dissolved organic ions, and nutrients in the water (also called xylem sap).
  • Xylem cells: The cells for transporting water are usually dead, and thus, the process of conduction occurs passively.
  • Passive transport: Due to passive transport, the conduction process does not require any form of energy.
  • Capillary action: The process of conduction of xylem sap against the gravity within the plant is known as capillary action. Also, the process occurs when water cohesion forces and surface tension moves the xylem sap upwards.
  • Additional support: As the plants grow taller, the xylem also develops to provide support to the plant and allow transport of water and minerals to the organs of the plant present at higher regions.

How does xylem works?

transpiration of water in xylem diagram
Figure 8:  transpiration of water in xylem. Credit: FeltyRacketeer6 – (diagram), CC BY-SA 4.0

How does xylem transport water? Cohesion-Adhesion theory is the hypothesis that attempts to explain how water travels upwards across the plant against gravity. Transpiration in plants is a major factor that drives water to move up to replace water that has been lost by evaporation. Xylem picks the water from the roots to transfer to other parts of the plants. Several cells are involved in the process of conduction or transportation of water.

Read: Plant Water Regulation Lesson (free tutorial)

Tracheary elements (including vessels and tracheids) are dead cells after reaching maturity. Therefore, they act passively for water transportation. The water reaches upwards from roots towards the stem and leaves on the basis of two factors: root pressure and transpirational pull.

  • Root pressure: Occurs due to osmosis (the movement of water from high concentration area to low concentration area) that lets the water from the soil or ground into the roots.
  • Transpirational pull: The surface tension pulls the water upwards within the xylem caused due to the loss of water through the transpiration process from the leaves.
The mode of transport is passive transport. For taller plants, though, the capillary action is coupled by transpiration, which is the loss of water by evaporation. The loss of water through transpiration leads to a high surface tension, which in turn, results in negative pressure in the xylem. Consequently, the water from the roots is lifted to as high as several meters from the ground towards the apical parts of the plant. 

Xylem Evolution

Around 400 million years ago, the xylem was developed in plants due to adaptation to environmental requirements. The production of food through photosynthesis is characterized by water uptake and carbon dioxide. When plants colonized the land, they developed a more advanced transport system that increases their chances of survival on the ground. Eventually, plants evolved advanced structures, such as the xylem vascular system. The water concentration n the plant reduced through the transpirational process (that occurs through stomata taking carbon dioxide in and water out). As explained in the previous section, this transpiration helped pull water in the plant body against gravity.

Developmental Process of Xylem

The development of the xylem is characterized by the bifacial lateral meristem cells and the vascular cambium that produces secondary xylem (as well as secondary phloem). Moreover, the development of xylem changes from one form to another. Different terms are used to describe the xylem’s development. They are exarch, endarch, mesarch, and centrarch.

  • Centrarch: The primary xylem develops outward from the cylinder produced in the middle of the stem; thus, metaxylem surrounds the protoxylem. For instance, several land plants have a centrarchid form of development.
  • Exarch: The xylem is developed inward from the outer side when the primary xylem is more than one in roots or stems. Therefore, the metaxylem is close to the center, whereas the protoxylem forms near the boundary. For example, the xylem of vascular plants has an exarch form of development.
  • Endarch: The xylem develops from the inner part and moves outward; thus, the protoxylem formed near the center, and the metaxylem formed close to the boundary. For instance, the seed plant’s stems show an endarch form of development.
  • Mesarch: Xylem develops in each direction from the center of the primary xylem’s strand. However, the metaxylem occupied both boundary and central areas leaving protoxylem in between. For instance, fern’s stems and leaves have a mesarch form of development.

The Xylem tissue is formed from meristem cells, such as those in the vascular cambium and the procambium. The phases of development and growth of xylem tissues can be distinguished into two phases. · The first phase is also known as the primary growth, which is characterized by the differentiation of primary xylem from cells that originated from procambium. The second phase, also known as secondary growth, is characterized by the generation of secondary xylem through a lateral meristem.

The growing and developing parts of the plant contain primary xylem consisting of metaxylem and protoxylem vessels. In the early phases of xylem development, the protoxylem changed into a metaxylem. These xylem vessels (protoxylem and metaxylem) can be differentiated on the basis of diameter and pattern of the cell wall (secondary) at the morphological level. Firstly, the protoxylem is a narrow vessel made up of small cells with cell walls containing thickenings such as helices or rings. The protoxylem cells develop and grow along with the elongation of roots or stems. Secondly, the metaxylem is larger in size with thickenings in scalariform (ladder-like) or pitted (sheet-like). After the period of elongation, when cells do not increase in size, the metaxylem completes its development. Thus, the xylem formed comprises dead cells that act as hollow strands to conduct water and dissolved minerals. According to research, xylem development can be enhanced through genetic engineering to get the desired results.


Try to answer the quiz below to check what you have learned so far about xylem.

Quiz

Choose the best answer. 

1. A common characteristic of a xylem that separates it from phloem
2. Conducts water from roots to other parts of the plant
3. Xylem tissue has xylem vessels
4. Xylem resembles a star by having "prongs" of xylem tissues
5. Characterized by having a secondary growth in stems

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References:

  • Myburg, A, Yadun, S. & Sederoff, R. (2013). Xylem Structure and Function. Wiley online library. 10.1002/9780470015902.
  • Foster, A.S. & Gifford, E.M. (1974). Comparative Morphology of Vascular Plants (2nd ed.).W.H. Freeman. 55–56. 978-0-7167-0712-7.
  • Taylor, T.N., Taylor, E.L., & Krings, M. (2009). Paleobotany, the Biology and Evolution of Fossil Plants (2nd ed.). Amsterdam; Boston: Academic Press. 207-212. 978-0-12-373972-
  • Růžička, K., Ursache, R., Hejátko, J., & Helariutta, Y. (2015). Xylem development – from the cradle to the grave. New phytologist foundation. 10.1111/nph.13383

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