Cellular Structure of Vascular Plants

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Xylem TracheidXylem Vessel ElementA recent article in Science Vol. 291 (26 January 2001) by N.M. Holbrook, M. Zwieniecki and P. Melcher suggests that xylem cells may be more than inert tubes. They appear to be a very sophisticated system for regulating and conducting water to specific areas of the plant that need water the most. This preferential water conduction involves the direction and redirection of water molecules through openings (pores) in adjacent cell walls called pits. The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together. One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. "When pectins swell, pores in the membranes are squeezed, slowing water flow to a trickle. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above." This remarkable control of water movement may allow the plant respond to drought conditions. See Article About The Forces Of Imbibition In PlantsPolysacharride Gums: Hydrogels & PhycocolloidsSpiral thickenings in the secondary walls of vessels and tracheids gives them the appearance of microscopic coils under high magnification with a light microscope.

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Magnified horizontal view (400x) of an inner perianth segment of a Brodiaea species in San Marcos showing a primary vascular bundle composed of several strands of vessels. The strands consist of vessels with spirally thickened walls that appear like minute coiled springs. Although this species has been called B. jolonensis by San Diego botanists for decades, it appears to be more similar to B. terrestris ssp. kernensis. This species contains at least 3 strands of vessels per bundle, while B. jolonensis only has one strand per bundle. The water-conducting xylem tissue in plant stems is actually composed of dead cells. In fact, wood is essentially dead xylem cells that have dried out. The dead tissue is hard and dense because of lignin in the thickened secondary cell walls. Lignin is a complex phenolic polymer that produces the hardness, density and brown color of wood. Cactus stems are composed of soft, water-storage parenchyma tissue that decomposes when the plant dies. The woody (lignified) vascular tissue provides support and is often visible in dead cactus stems.
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Left: Giant saguaro (Carnegiea gigantea) in northern Sonora, Mexico. The weight of this large cactus is largely due to water storage tissue in the stems. Right: A dead saguaro showing the woody (lignified) vascular strands that provide support for the massive stems.See Article About HardwoodsSee Specific Gravity Of WoodPhloem tissue conducts carbohydrates manufactured in the leaves downward in plant stems. It is composed of sieve tubes (sieve tube elements) and companion cells. The perforated end wall of a sieve tube is called a sieve plate. Thick-walled fiber cells are also associated with phloem tissue.In dicot roots, the xylem tissue appears like a 3-pronged or 4-pronged star. The tissue between the prongs of the star is phloem. The central xylem and phloem is surrounded by an endodermis, and the entire central structure is called a stele.
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Microscopic view of the root of a buttercup (Ranunculus) showing the central stele and 4-pronged xylem. The large, water-conducting cells in the xylem are vessels. In dicot stems, the xylem tissue is produced on the inside of the cambium layer. Phloem tissue is produced on the outside of the cambium. The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant (Linum usitatissimum) are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids. The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. This remarkable division includes Ephedra (Mormon tea), Gnetum, and the amazing Welwitschia of Africa"s Namib Desert.See Article About WelwitschiaPine stems also contain bands of cells called rays and scattered resin ducts. Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel. The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries. This is why wood with prominent rays often splits along the rays. In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is called an annual ring. By counting the rings (dark bands of summer xylem in pine wood), the age of a tree can be determined. Other data, such as fire and climatic data, can be determined by the appearance and spacing of the rings. Some of the oldest bristlecone pines (Pinus longaeva) in the White Mountains of eastern California have more than 4,000 rings. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill.
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Microscopic view of a 3-year-old pine stem (Pinus) showing resin ducts, rays and three years of xylem growth (annual rings).
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A cross section of loblolly pine wood (Pinus taeda) showing 18 dark bands of summer xylem (annual rings).See Article About The Anatomy Of WoodSee Resin Ducts In Poison Oak StemSee World"s Oldest Living ShrubAngiosperms typically have both tracheids and vessels. In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods. Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods.See Article About HardwoodsSee Specific Gravity Of WoodThe following illustrations and photos show American basswood (Tilia americana), a typical ring-porous hardwood of the eastern United States:
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A cross section of the stem of basswood (Tilia americana) showing large pith, numerous rays, and three distinct annual rings.
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A cross section of the stem of basswood (Tilia americana) showing pith, numerous rays, and three distinct annual rings. The large spring xylem cells are vessels.
Lack Of Visible Annual Rings In Tropical Trees?In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree (Miliusa velutina) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.Poussart, P.M., Myneni, S.C.B., Lanzirotti, A., et al. 2006. Geophysical Research Letters 3: L17711.
Anatomy Of Monocot StemsMonocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings. They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue. Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn (Zea mays):
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A cross section of the stem of corn (Zea mays) showing parenchyma tissue and scattered vascular bundles. The large cells in the vascular bundles are vessels. Unlike most monocots, palm stems can grow in girth by an increase in the number of parenchyma cells and vascular bundles. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem." New vascular bundles and parenchyma tissue are added as the stem grows in diameter.
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The massive trunk of this Chilean wine palm (Jubaea chilensis) has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems.Palm WoodThe scattered vascular bundles containing large (porous) vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm.
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Cross section of the trunk of the native California fan palm (Washingtonia filifera) showing scattered vascular bundles. The large cells (pores) in the vascular bundles are vessels.
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The trunk of a California fan palm (Washingtonia filifera) in Palm Canyon, Anza-Borrego State Park. The palm was washed down the steep canyon during the flash flood of September 2004. The fibrous strands are vascular bundles composed of lignified cells.
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Right: Cross section of the trunk of a California fan palm (Washingtonia filifera) showing scattered vascular bundles that appear like dark brown dots. The dot pattern also shows up in the petrified Washingtonia palm (left). The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood (right) is caused by the larva of the bizarre palm-boring beetle (Dinapate wrightii) shown at bottom of photo. An adult beetle is shown in the next photo.
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An adult palm-boring beetle (Dinapate wrightii)Read About The Palm-Boring BeetleRead About The Anatomy Of WoodRead About Plant Textile FibersGo To Fossilized Plants PageBamboo Wood
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A petrified trunk from the extinct tree fern Psaronius brasiliensis. The central stele region contains arc-shaped vascular bundles of xylem tissue. The stem is surrounded by leaf bases which formed the leaf crown of this fern, similar to present-day Cyathea tree ferns of New Zealand. This petrified stem has been cut and polished to make a pair of bookends.
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A well-preserved stem section from the extinct tree fern Psaronius brasiliensis. Note the central stele region containing arcs of xylem tissue (vascular bundles). The structure of this stem is quite different from the concentric growth rings of conifers and dicots, and from the scattered vascular bundles of palms.

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