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Xylem is a type of plant tissue which is used to transfer water and nutrients. Xylem is commonly found in higher plants. The plants consisting xylem are also called vascular plants and distinguish them from other non-vascular plants. The term ‘XYLEM ’was first introduced by Carl Negali in 1858. Xylem is derived from Greek word Xylon meaning wood.
Biologists defined the xylem as a specialized tissue used for transportation of water and minerals in vascular plants. It also helps in storage and provide support to plants. It is made up of several types of cells and comprises complex systems to transport water and minerals.
Xylem and Phloem
|It helps in transportation of water and minerals.||It is mainly involved in transporting glucose, and other organic solutes.|
|Transfer minerals from root to other parts of the plant.||Transfer organic molecules from leaves to other parts of the plant.|
|The transportation occur in unidirectional way.||Transport in both directions; bidirectional transport.|
|Water is transported through transpiration pull.||It use ATP molecules to transport glucose molecules.|
|It is located in the middle of vascular bundle.||Located near the periphery of vascular bundles.|
Role of Xylem in Vascular Plants
The major role of xylem fibers is to transport water and minerals. The presence of vascular bundles helps the vascular plants to grow higher than non-vascular plants. Xylem is also responsible for the rigid form of the plant and provides support to the plant.
Role of Phloem in Vascular Plants
Phloem has the main function of transporting nutrients including proteins, sugar, and organic molecules.
The Xylem of Angiosperms and other Vascular Plants
Angiosperms are also known as flowering plants. Another group of vascular plants is named gymnosperms that do not produce flowers and produce naked seeds. Both the group are combined called vascular plants or higher plants.
The structure and composition of the xylem also differ in gymnosperms and angiosperms. The angiosperms contain the xylem tissue with xylem vessels whereas the xylem vessels are absent in gymnosperms. Xylem of gymnosperms contain tracheids.
Both xylem vessels and tracheids become non-living, hollow structures and lose their protoplast at maturity. As compare to the tracheid, xylem vessels have thinner cell walls primarily made of lignin.
The xylem vessel is made up of vessel elements that have partially or wholly dissolved common end walls. Due to the wider diameter, the vessels conduct more water than the tracheid.
Xylem: Monocot vs Dicot
Flowering plants are further classified into two major groups: (1) monocots, and (2) Dicots. Based on the number of cotyledons, they are divided into two groups. The plants having only one cotyledon are called monocots whereas plants having two cotyledons are known as dicots.
The structure of the xylem also differs in monocots and dicots. For instance, a dicot root has xylem with a star-like appearance whereas monocots have alternating xylem and phloem tissues. The xylem vessels are also different in monocots and dicots, the xylem of monocots are oval-shaped whereas the dicots have polygonal or angular xylem vessels. Apart from the root, the stem of monocots consists of scattered vascular bundles whereas the stem of dicots is arranged in a ring pattern.
Types of Xylem
Based on the structure, function, and development the xylem can be classified into two types, primary and secondary xylem.
i. Primary xylem: The primary xylem starts growing primarily at the tips of stems, roots, and flower buds. The plant grows taller with the help of the primary xylem therefore it is named as primary xylem.
ii. Secondary xylem: The secondary xylem helps the plant to get wider over time. The wide tree trunks are the common examples of secondary xylem. The dark rings formed by the secondary xylem are used to determine the age of the trees.
Structure of Xylem
Xylem is made of four types of elements:
(1) xylem vessels
(3) xylem fiber, and
(4) xylem parenchyma
(1) Xylem vessels: The flowering plants or angiosperms consist of xylem vessels that are long cylindrical structures. A large cavity is present in the lignified wall of the vessel. Xylem vessels become dead at maturity and lose their protoplasm. Vessels help in the conduction of water, minerals and also provide mechanical strength to the plant.
(2) Tracheids: Tracheids are also present in gymnosperms. They have a thicker wall as compare to vessels that are lignified and lack protoplasm. They are also dead cells that perform water and mineral transportation.
(3) Xylem fibers: The dead cells having a central lumen and lignified walls are called xylem fibers. They are responsible for the transportation and mechanical support.
(4) Xylem parenchyma: Characteristics of xylem parenchyma
• They are only living cells of the xylem.
• The cell wall is thin and composed of cellulose
• It has a prominent nucleus and protoplast.
• A large vacuole is present.
• Divided into septa that are composed of crystal-containing parenchyma cells.
• Outgrowths called “tyloses” are formed in the vessels.
• The tyloses are further differentiated into sclereids.
The major functions of xylem parenchyma include-
• Conduction of water in an upward direction.
• Helps in the storage of food nutrients in the form of fats, tannins, and starch
• The tyloses connect parenchyma cells of the xylem to vessels.
• Tyloses also protect vascular tissues in the condition of drought or infection.
Characteristics of Xylem Tissue
• The structure of the xylem is composed of different types of xylem cells including fibers, parenchyma cells, and tracheary elements.
• Xylem cells get support from the parenchyma cells • The hollow, dead strands are called tracheary elements that let water and minerals flow through them.
• Perforation plates present in vessels connect different vessel elements into one continuous sheet.
• Under the microscope, the xylem resembles a star-shaped structure.
The major functions of the xylem include transportation of minerals and providing mechanical support to the plant. Some major functions of xylems are described below-
Support: Mechanical support and strength are provided by the xylem to plant, it also helps to maintain the plant’s structure and prevents plants from bending.
Xylem sap: it helps the flow of water, dissolved organic ions, and nutrients in the water. The structure of xylem sap appears like a long tube.
Xylem cells: Helps in water transduction, they lack protoplast and remain dead in maturity, therefore, conduct passive transport of water and minerals.
Passive transport: Minerals are transported by passive transport that does not require energy.
Capillary action: Capillary action is the process of conduction of xylem sap against gravity. It helps in the upwards movement of water.
How does the Xylem Works?
The xylem works on the principle of Cohesion- Adhesion theory. This hypothesis explains the upwards movement of water against gravity. Another major factor is transpiration that drives water to move up and replace water that has been lost by transpiration. Root absorbs water and the xylem picks the water to transfer it to all parts of the plant. There are two major factors that help in the upward movement of water: root pressure and transpiration pull.
Root Pressure: It occurs due to osmosis and allows the movement of water from the soil into the root.
Transpiration Pull: Surface tension helps in the upward movement of water within the xylem. Transpiration causes loss of water which is replaced by transpiration pull.
Xylem Evolution: The xylem was evolved 400 million years ago. The adaptation due to the environmental requirements develops vascular bundles in plants. Water and carbon dioxide are essential elements for photosynthesis. The transport system is first developed in the terrestrial plants to increase the survival chances of these plants.
Later, the structure evolved into the xylem vascular system that consists of different types of cells.
Developmental Process of Xylem
Development of the xylem can be explained by the help of different terms such as exarch, endarch, mesarch, and centrarch.
• Centrarch: Several terrestrial plants use a centrarchid form of development in which a cylinder is produced in the middle of the stem from which the xylem develops. Therefore metaxylem surrounds the protoxylem.
• Exarch: Most of the vascular plants adopt an exarch form of development in which the inner part develops first and moves outward. In this type of development, the protoxylem was found near the center and the metaxylem formed toward the boundary.
• Mesarch: several ferns have a mesarch form of development in which the xylem develops in each direction. In this type of development, the protoxylem leaves in the center surrounded by a metaxylem from both sides.
The xylem tissues mainly developed from meristem cells. The growth of xylem tissues can be described by two phases that are primary growth and secondary growth. The primary growth involves the differentiation of primary xylem from cells of procambium whereas generation of secondary xylem through a lateral meristem occurs in the second phase.
The primary xylem is mainly responsible for growth and development that consists of metaxylem and protoxylem vessels. The protoxylem changes into a metaxylem in the early phase of xylem development. Based on the diameter and pattern of the cell wall, xylem vessels are differentiated into protoxylem and metaxylem.
The metaxylem is larger in size as compare to the protoxylem that develops along with the elongation of roots. Some research proved that the development process of xylem can be enhanced through genetic engineering.
- Soil Rather Than Xylem Vulnerability Controls Stomatal Response to Drought. Trends Plant Sci . 2020 Sep;25(9):868-880.
- Positive pressure in xylem and its role in hydraulic function. New Phytol . 2021 Apr;230(1):27-45.
- Functional Status of Xylem Through Time. Annu Rev Plant Biol . 2019 Apr 29;70:407-433.