structure and function of xylem and phloem pdf

Structure And Function Of Xylem And Phloem Pdf

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Parenchyma is the precursor of all the other tissues.

Xylem is the complex, dead and permanent tissue responsible for carrying nutrients and water, whereas phloem is the soft and permanent tissue play its role in transporting the food and other organic material produced by the green parts especially leaves by the process of photosynthesis. Xylem and phloem are the two types of vascular tissues , present in plants and together constitute vascular bundles.

Vascular Systems of Plants Xylem and phloem make up the big transportation system of vascular plants. As you get bigger, it is more difficult to transport nutrients, water, and sugars around your body. You have a circulatory system if you want to keep growing.

Xylem and phloem

These two tissues extend from the leaves to the roots, and are vital conduits for water and nutrient transport. In a sense, they are to plants what veins and arteries are to animals. The structure of xylem and phloem tissue depends on whether the plant is a flowering plant including dicots and monocots or a gymnosperm polycots. The terms dicot, monocot and polycot are summarized in the following table. Class Monocotyledoneae: Monocots Flower parts in 3's or multiple of 3's; one cotyledon inside seed; parallel leaf venation; includes Lilium , Amaryllis , Iris , Agave , Yucca , orchids, duckweeds, annual grasses, bamboos and palms.

Class Dicotyledoneae: Dicots Flower parts in 4's or 5's; 2 cotyledons inside seed; branched or net leaf venation; contains the most species of flowering herbs, shrubs and trees; includes roses Rosa , buttercups Ranunculus , clover Trifolium , maple Acer , basswood Tilia , oak Quercus , willow Salix , kapok Ceiba and many more species. Some of the coniferous genera division Coniferophyta are the most important timber trees in the world. Since these species have several cotyledons inside their seeds, they are conveniently referred to as polycots.

In dicot stems, the cambium layer gives rise to phloem cells on the outside and xylem cells on the inside. All the tissue from the cambium layer outward is considered bark, while all the tissue inside the cambium layer to the center of the tree is wood. Xylem tissue conducts water and mineral nutrients from the soil upward in plant roots and stems. It is composed of elongate cells with pointed ends called tracheids, and shorter, wider cells called vessel elements.

The walls of these cells are heavily lignified, with openings in the walls called pits. Tracheids and vessels become hollow, water-conducting pipelines after the cells are dead and their contents protoplasm has disintegrated. The xylem of flowering plants also contains numerous fibers, elongate cells with tapering ends and very thick walls.

Dense masses of fiber cells is one of the primary reasons why angiosperms have harder and heavier wood than gymnosperms. This is especially true of the "ironwoods" with wood that actually sinks in water. 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.

But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above. Magnified horizontal view x 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. This species contains at least 3 strands of vessels per bundle, while B. T he 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. 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. 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. I n 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.

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. 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. See Article About Welwitschia P ine 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, rings.

Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill. Microscopic view of a 3-year-old pine stem Pinus showing resin ducts, rays and three years of xylem growth annual rings. 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 Hardwoods See Specific Gravity Of Wood T he following illustrations and photos show American basswood Tilia americana , a typical ring-porous hardwood of the eastern United States: A cross section of the stem of basswood Tilia americana showing large pith, numerous rays, and three distinct annual rings.

The large spring xylem cells are vessels. 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. 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. Geophysical Research Letters 3: L Anatomy Of Monocot Stems M onocot 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 : 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. 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. 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 Wood T he scattered vascular bundles containing large porous vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm.

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. The palm was washed down the steep canyon during the flash flood of September The fibrous strands are vascular bundles composed of lignified cells. 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.

Phloem: Cell Types, Structure, and Commercial Uses

These two tissues extend from the leaves to the roots, and are vital conduits for water and nutrient transport. In a sense, they are to plants what veins and arteries are to animals. The structure of xylem and phloem tissue depends on whether the plant is a flowering plant including dicots and monocots or a gymnosperm polycots. The terms dicot, monocot and polycot are summarized in the following table. Class Monocotyledoneae: Monocots Flower parts in 3's or multiple of 3's; one cotyledon inside seed; parallel leaf venation; includes Lilium , Amaryllis , Iris , Agave , Yucca , orchids, duckweeds, annual grasses, bamboos and palms. Class Dicotyledoneae: Dicots Flower parts in 4's or 5's; 2 cotyledons inside seed; branched or net leaf venation; contains the most species of flowering herbs, shrubs and trees; includes roses Rosa , buttercups Ranunculus , clover Trifolium , maple Acer , basswood Tilia , oak Quercus , willow Salix , kapok Ceiba and many more species.

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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. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.

Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant. The first fossils that show the presence of vascular tissue date to the Silurian period, about million years ago. The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem.

Difference Between Xylem and Phloem

Stem & Root Anatomy

Xylem is one of the two types of transport tissue in vascular plants , phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The most distinctive xylem cells are the long tracheary elements that transport water.

Phloem is the vascular tissue in charge of transport and distribution of the organic nutrients. The phloem is also a pathway to signaling molecules and has a structural function in the plant body. It is typically composed of three cell types: sieve elements, parenchyma, and sclerenchyma. The sieve elements have the main function of transport and typically have lost their nuclei and other organelles in the course of their specialization. Hence, the sieve elements rely on specialized neighboring parenchyma cells to sustain all of their physiological function and activities. All cell types of the phloem may vary morphologically and in their distribution in the tissue, and this diversity is taxonomic and functionally informative.


- Phloem tubes carry sugar & other organic nutrients made by plant from the leaves to the rest of the plant. Structure of the phloem tissue. This is a long tube that.


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