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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.
Between the xylem and phloem is a meristem called the vascular cambium. This tissue divides off cells that will become additional xylem and phloem. This growth increases the girth of the plant, rather than its length. As long as the vascular cambium continues to produce new cells, the plant will continue to grow more stout.
A vessel element or vessel member (also called a xylem vessel) [1] is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are found in most angiosperms ( flowering plants ) and in some gymnosperms such as cycads and Ephedra , but absent in conifers .
Primary and secondary roots in a cotton plant. Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves. [1] Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is
Tracheids were the main conductive cells found in early vascular plants. In the first 140–150 million years of vascular plant evolution, tracheids were the only type of conductive cells found in fossils of plant xylem tissues. [5] Ancestral tracheids did not contribute significantly to structural support, as can be seen in extant ferns. [6]
The xylem consists of vessels in flowering plants and of tracheids in other vascular plants. Xylem cells are dead, hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall usually contains the polymer lignin. The phloem, on the other hand, consists of living cells called sieve-tube members ...
The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the aerial parts of the plant. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessel members and tracheids.
In the primary pit, the primordial pit provides an interruption in the primary cell wall that the plasmodesmata can cross. The primordial pit is the only aperture in the otherwise continuous primary cell wall. [3] Pit pairs are a characteristic feature of xylem, as sap flows through the pits of xylem cells. [4]