Xylem
Say it: ZY-lum
Xylem is the vascular tissue of plants that carries water and dissolved minerals upward from the roots to the stems and leaves.
Definition
What is xylem?
Xylem is one of the two vascular tissues of plants, alongside phloem. It forms a continuous system of tubes running from the roots through the stem to the leaves, carrying water and the minerals dissolved in it. Its water-conducting cells, tracheids and vessel elements, are dead when mature: they undergo programmed cell death, leaving hollow tubes with thick walls stiffened by lignin. Water passes between tracheids through thin spots called pits, and flows through open end walls in vessels stacked into long pipes. Living xylem parenchyma cells and tough fibers complete the tissue.
Water moves up the xylem passively, without the plant spending energy. As water evaporates from leaves during transpiration, it creates tension that pulls the water column upward, while the cohesion of water molecules to each other and their adhesion to the walls keep the column unbroken. This is the cohesion-tension mechanism. If an air bubble forms, the conduit can be blocked, a problem called embolism.
In trees, the vascular cambium adds new xylem each year, and this secondary xylem is wood. Seasonal changes in the size of xylem cells produce the annual growth rings used to estimate a tree's age.
Word parts
Where does the word “xylem” come from?
- xyl-
- woodGreek xylon
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Terms that go with it
- PhloemPhloem is the living vascular tissue of plants that carries sugars and other organic nutrients from leaves to the parts that use or store them.
- TranspirationTranspiration is the escape of water vapor from a plant, mostly through open stomata in its leaves, which pulls more water up through the xylem.
- StomataStomata are tiny pores in the epidermis of leaves and stems, each opened and closed by two guard cells to control gas exchange and water loss.
- ApoptosisApoptosis is programmed cell death, a controlled process in which a cell dismantles itself in an orderly way without harming its neighbors.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Biology 2e, 30.2 Stems
- OpenStax Biology 2e, 30.5 Transport of Water and Solutes in Plants
- Serk et al. (2015), Cooperative lignification of xylem tracheary elements, Plant Signaling & Behavior
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