Xylem And Phloem

Xylem Definition

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 the tissue is made up of several kinds of cells. Tracheids or treachery 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 Definition

 

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.