In the Carl Negali introduced the term xylem. Wood is a popular example of xylem. What is xylem? It also provides storage and support to the plant Myburg. 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. 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. What is the role of xylem in a vascular plant? The phloem of vascular plants is responsible for transporting nutrients including, sugar , proteins , and organic molecules that help plants to remain alive and reproduce. 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. 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. Angiosperms may be grouped into two major groups: 1 the monocots e. 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. 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. Apart from the roots, the dicots and the monocots have apparent differences in their stems.
The vascular bundles i. 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.
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. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:. Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil.
In extreme circumstances, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree. Capillary action or capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube capillary. Capillarity occurs due to three properties of water:.
On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree. This video provides an overview of the important properties of water that facilitate this movement:. The c ohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants.
Cohesion-tension essentially combines the process of capillary action with transpiration , or the evaporation of water from the plant stomata.
Transpiration is ultimately the main driver of water movement in xylem. The cohesion-tension model works like this:. Here is a bit more detail on how this process works: Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis.
The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure.
Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap.
The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
This video provides an overview of the different processes that cause water to move throughout a plant use this link to watch this video on YouTube , if it does not play from the embedded video :.
This table explains what is transported by the xylem and phloem:. Mature xylem consists of elongated dead cells, arranged end to end to form continuous vessels tubes. Mature xylem vessels:. Phloem consists of living cells arranged end to end. Transport in the xylem is a physical process.
It does not require energy. Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as:. Transport in the phloem is therefore both up and down the stem. Transport of substances in the phloem is called translocation. Phloem consists of living cells. The cells that make up the phloem are adapted to their function:.
The xylem and phloem are distributed differently in roots and stems.
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