Last Updated on September 16, 2020 by Sagar Aryal
Xylem is a vascular tissue that transports water and dissolved minerals absorbed from the roots to the rest of the plant.
- The term xylem is derived from the Greek word ‘xylon’ which means wood as the best-known xylem tissues are found in the woody part of the stem.
- When observed under the microscope, xylem tissue has a star-like appearance.
- The cells in this tissue are mostly dead cells, and the cells are lignified. The cell wall is thick and made up of lignin which aids in its function of providing support.
- Xylem is present at the center of vascular bundles where the transport of water and mineral is unidirectional. The quantity of xylem is more than the phloem in these bundles.
- Xylem tissue is made up of several kinds of cells. Tracheids or trachery elements are specialized, water-conducting cells that help in transport as well as provide physical support.
- The next groups of cells are vessel elements that are shorter than tracheids but also help in transport. Vessel membranes have perforations through which the water and dissolved minerals are conducted.
- Vessel elements are found in flowering plants where they are connected to form one continuous vessel. These elements are found not in gymnosperms.
- Besides, xylem also contains parenchyma that provides support to the plant in the form of long fibers in the soft parts of the plant.
- The initial development of xylem occurs from the active root cells and apical meristem, which give rise to primary xylem.
- In hard and woody plants, secondary xylem develops as rings around the primary xylem as the plant expands in girth.
- Ultimately, the primary xylem dies and loses its conducting function but acts as a skeleton providing physical support.
- The secondary xylem continues to function as a water-conducting tissue.
- In mature and woody plants, the wood or xylem is differentiated into heartwood and sapwood. The heart represents the primary xylem and provides mechanical strength, whereas the sapwood is the secondary xylem that conducts water and minerals.
- The transport of water and minerals in the xylem is a passive process where no energy is required for the transport of these substances.
- The functions of xylem include replacing the water lost during photosynthesis and transpiration by absorbing it from the root and providing physical support.
Phloem is a vascular tissue that transports soluble organic compounds prepared during photosynthesis from the green parts of the plant to the rest of the plant.
- The term phloem is taken from the Greek word ‘phloios’ which means bark, as the phloem makes up most of the bulk of the bark of the plants.
- Phloem tissue is present towards the periphery of the vascular bundles and is less in quantity than the xylem tissue.
- Phloem, like xylem, is comprised of several specialized cells like sieve tubes, companion cells, phloem fibers, and phloem parenchyma. These cells are also living cells and are not lignified.
- Sieve tubes are columns of sieve-tube cells with perforations on the lateral wall through which the food substances travel.
- Phloem fibers are long flexible cells that make up the soft fibers in plants like hemp and flax.
- Phloem parenchyma consists of companion cells and albuminous cells that function to provide support to the sieve elements and help in the termination of sieve tubes in the leaf veinlets.
- Besides, sclerenchyma is another group of cells that provide support and stiffness to the phloem tissue. The tissue has two types of cells; fibers and sclereids.
- Fibers are long and flexible with a narrow lumen, whereas the sclereids are shorter irregular cells that add strength to the tissue.
- The primary phloem is formed from the apical meristem of the shoot and the root during the developmental stages of the plant. The primary phloem can either be protophleom or metaphloem.
- The sieve tubes of protophloem are unable to stretch with the elongating tissues and are destroyed as the plant matures. However, tube cells of metaphloem mature after elongation and thus survive the maturation phase to be converted into fibers.
- The cells of the metaphloem function until the secondary phloem is formed in plants with cambium.
- The transport of food, including sugar and amino acids from leaves to the other parts of the plant, is the primary function of the phloem. The transport in the phloem is bidirectional where the food can move both up and down the tissues.
- The transport is an active process where energy is required for the movement of the food particles.
Key Differences (Xylem vs Phloem)
Basis for Comparison
|Definition||Xylem is a vascular tissue that transports water and dissolved minerals absorbed from the roots to the rest of the plant.||Phloem is a vascular tissue that transports soluble organic compounds prepared during photosynthesis from the green parts of the plant to the rest of the plant.|
|Terms||The term xylem is derived from the Greek word ‘xylon’ which means wood as the best-known xylem tissues are found in the woody part of the stem.||The term phloem is taken from the Greek word ‘phloios’ which means bark, as the phloem makes up most of the bulk of the bark of the plants.|
|Location||Xylem is mainly located in the center of the vascular bundles.||Phloem is mainly localized towards the periphery of the vascular bundles.|
|Xylem forms most of the bulk of the wood.||Phloem forms most of the bulk of the bark.|
|Found in||Xylem tissues are found in leaves, roots, and stems.||Phloem tissues are found in stems and leaves which later grow in the roots, fruits, and seeds.|
|Composed of||Xylem tissue is composed of xylem vessels, fibers, and tracheids.||Phloem tissue is composed of like sieve tubes, companion cells, phloem fibers, and phloem parenchyma.|
|Fibers||Xylem fibers are robust and longer.||Phloem fibers are flexible in shorter.|
|Cells||The cells of the xylem tissue are dead cells except for the parenchyma cells.||The cells of the phloem tissue are living cells except for the blast fibers.|
|The cell wall of the cells in the xylem is thick-walled.||The cell wall of the cells of the phloem is thin-walled.|
|Lignified cell walls are present in the xylem.||The cell wall is not lignified.|
|Quantity||The quantity of xylem tissue in the vascular bundles is more than the phloem tissue.||The quantity of phloem tissue is comparatively less in the vascular tissue.|
|Tyloses||Tyloses are formed in the xylem.||Tylose is not formed in the phloem.|
|Conducive cells||Two types of conducive cells are present in xylem; tracheids and vessels.||Only one type of conducive cell is present in phloem; sieve tubes.|
|The conducive tissues consist of dead cells.||The conducive tissues consist of living cells.|
|Differentiation||In mature plants, xylem is differentiated into heartwood and sapwood.||No such differentiation is observed in the phloem.|
|Function||The primary function of xylem is to transport water and dissolved minerals from the root to different parts of the plant.||The primary function of the phloem is to transport the prepared sugars from the leaves to different parts of the plant.|
|Direction||The transport by xylem is unidirectional; the water and mineral are only moved up from the roots.||The transport by phloem is bidirectional; the food can travel both up and down the plant.|
|Mechanical support||Xylem also aids in providing physical support to the plant.||Phloem is not involved in mechanical support.|
References and Sources
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- 3% – https://www.britannica.com/science/phloem
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- 1% – https://nigerianscholars.com/tutorials/plant-form-and-physiology/movement-of-water-and-minerals-in-the-xylem/
- 1% – https://en.wikipedia.org/wiki/Phloem
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- <1% – https://www.researchgate.net/publication/261401805_Development_of_Intra-_and_Interxylary_Secondary_Phloem_in_Coccinia_indica_Cucurbitaceae
- <1% – https://www.qsstudy.com/biology/describe-structures-functions-xylem-tissue
- <1% – https://www.dictionary.com/browse/xylem
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- <1% – https://www.bbc.co.uk/bitesize/guides/zps82hv/revision/1
- <1% – https://vivadifferences.com/difference-between-primary-xylem-and-secondary-xylem/
- <1% – https://simple.wikipedia.org/wiki/Phloem
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- <1% – https://biologydictionary.net/vascular-tissue/
- <1% – http://blogs.ubc.ca/biol343/cell-tissue-types-2/