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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]
Two kinds of cell are involved in xylem transport: tracheids and vessel elements. [2] [3] [4] Vessel elements are the building blocks of vessels, the conducting pathways that constitute the major part of the water transporting system in flowering plants. Vessels form an efficient system for transporting water (including necessary minerals) from ...
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 vascular element, found in angiosperms, is the vessel element. Vessel elements are joined end to end to form vessels in ...
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 ...
Cross section of celery stalk, showing vascular bundles, which include both phloem and xylem Detail of the vasculature of a bramble leaf Translocation in vascular plants. Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem ...
The group was described as a subdivision of the division Tracheophyta by Harlan Parker Banks in 1968 under the name Rhyniophytina. The original definition was: "plants with naked (lacking emergences), dichotomizing axes bearing sporangia that are terminal, usually fusiform and may dehisce longitudinally; they are diminutive plants and, in so far as is known, have a small terete xylem strand ...
In herbaceous plants, it occurs in the vascular bundles which are often arranged like beads on a necklace forming an interrupted ring inside the stem. In woody plants, it forms a cylinder of unspecialized meristem cells, as a continuous ring from which the new tissues are grown.
The rhyniophytes included "protracheophytes", which were precursors to vascular plants (e.g., Horneophyton, Aglaophyton); basal tracheophytes (e.g., Stockmansella, Rhynia gwynne-vaughanii); and plants allied to the lineages that led to the living club-mosses and allies as well as ferns and seed plants (e.g., Cooksonia species).