| Xylem |
Articles about Xylem |
Information AboutXylem |
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| plant anatomy | |
| plant physiology | |
| plant cells | |
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PHYSIOLOGY OF XYLEM The xylem transports water from the root up the plant. The xylem is mainly responsible for the transportation of water and mineral nutrients throughout the plant. Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well. This transport is not powered by energy spent by the tracheary elements themselves, which are dead at maturity and no longer have living contents. Two phenomena cause xylem sap to flow:
ANATOMY OF XYLEM Xylem can be found:
Note that, in transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain ''primary xylem'' only. The most distinctive s and Vessel Element s. However, the xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells, such as parenchyma, in addition to those that serve to transport water. PRIMARY AND SECONDARY XYLEM Primary xylem, also known as ''Protoxylem'', is the xylem that is formed during primary growth from Procambium . Secondary xylem, also known as ''Metaxylem'', is the xylem that is formed during secondary growth from Vascular Cambium . Secondary xylem is also found in members of the " Gymnosperm " groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta . The two main groups in which secondary xylem can be found are: #'''. #. In the remainder of the angiosperms this secondary xylem may or may not be present, this may vary even within a species, depending on growing circumstances. In view of the size of this group it will be no surprise that no absolutes apply to the structure of secondary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as Hardwood . EVOLUTION OF XYLEM Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the ''protoxylem'' (first-formed xylem) of all living groups of plants. Several groups of plants later developed pitted tracheid cells, apparently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the ''metaxylem'' (following the ''protoxylem''). In most plants, pitted tracheids function as the primary transport cells. The other type of tracheary element, besides the tracheid, is the Vessel Element . Vessel elements are joined by perforations into vessels. In vessels, water travels by ''bulk flow'', like in a pipe, rather than by diffusion through cell membranes. The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms SEE ALSO
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