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VASCULAR AND MECHANICAL TISSUES OF PLANTS.
COMPLEX TISSUES OF PLANTS – XYLEM AND PHLOEM.
VASCULAR BUNDLES
Ferns, Gymnosperms and Angiosperms are all terrestrial plants. In the previous chapter we learnt that cell division is important for growth and reproduction. Tissues therefore arise as a result of a cell or a group of cells dividing, thus giving rise to a large number of cells. In flowering plants these cells undergo changes that change the structure and function of the cell; differentiation has take place. As a result of differentiation, a cell or group of cells can have a specialized function.
http://www.youtube.com/watch?v=U87MdGeMHRI
Simple Tissues
There are three basic types, named for the type of cell that makes up their composition.
1. Parenchyma cells form parenchyma tissue. Parenchyma cells are the most abundant of cell types and are found in almost all major parts of higher plants (we will discuss higher plants later in the tutorial). These cells are basically sphere shaped when they are first made. However, these cells have thin walls, which flatten at the points of contact when many cells are packed together. Generally, they have many sides with the majority having 14 sides. These cells have large vacuoles and may contain various secretions including starch, oils, tannins, and crystals. Some parenchyma cells have many chloroplasts and form the tissues found in leaves. This type of tissue is called chlorenchyma. The chief function of this type of tissue is photosynthesis, while parenchyma tissues without chloroplasts are generally used for food or water storage. Additionally, some groups of cells are loosely packed together with connected air spaces, such as in water lilies, this tissue is called aerenchyma tissue. These type of cells can also develop irregular extensions of the inner wall which increases overall surface area of the plasma membrane and facilitates transferring of dissolved substances between adjacent cells. Parenchyma cells can divide if they are mature, and this is vital in repairing damage to plant tissues. Parenchyma cells and tissues comprise most of the edible portions of fruit.
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Collenchyma cells form collenchyma tissue. These cells have a living protoplasm, like parenchyma cells, and may also stay alive for a long period of time. Their main distinguishing difference from parenchyma cells is the increased thickness of their walls. In cross section, the walls looks uneven. Collenchyma cells are found just beneath the epidermis and generally they are elongated and their walls are pliable in addition to being strong. As a plant grows these cells and the tissues they form, provide flexible support for organs such as leaves and flower parts. Good examples of collenchyma plant cells are the ‘strings’ from celery that get stuck in our teeth.
3. Sclerenchyma cells form sclerenchyma tissue. These cells have thick, tough secondary walls that are imbedded with lignin. At maturity, most sclerenchyma cells are dead and function in structure and support. Sclerenchyma cells can occur in two forms:
1. Sclereids are sclerenchyma cells that are randomly distributed throughout other tissues. Sometimes they are grouped within other tissues in specific zones or regions. They are generally as long as they are wide. An example, would be the gritty texture in some types of pears. The grittiness is due to groups of sclereid cells. Sclereids are sometimes called stone cells.
2. Fibers are sometimes found in association with a wide variety of tissues in roots, stems, leaves and fruits. Usually fiber cells are much longer than they are wide and have a very tiny cavity in the center of the cell. Currently, fibers from over 40 different plant families are used in the manufacture of textiles, ropes, string and canvas goods to name a few.
Complex Tissues
Tissues composed of more than one cell type are generically referred to as complex tissues. Xylem and phloem are the two most important complex tissues in a plant, as their primary functions include the transport of water, ions and soluble food substances throughout the plant. While some complex tissues are produced by apical meristems, most in woody plants are produced by the vascular cambium and is often referenced as vascular tissue. Other complex tissues include the epidermis and the periderm. The epidermis consists primarily of parenchyma-like cells and forms a protective covering for all plant organs. The epidermis includes specialized cells that allow for the movement of water and gases in and out of the plant, secretory glands, various hairs, cells in which crystals are accumulated and isolated, and other cells that increase absorption in the roots. The periderm is mostly cork cells and therefore forms the outer bark of woody plants. It is considered to be a complex tissue because of the pockets of parenchyma cells scattered throughout.
Xylem
Xylem is an important plant tissue as it is part of the ‘plumbing’ of a plant. Think of bundles of pipes running along the main axis of stems and roots. It carries water and dissolved substances throughout and consists of a combination of parenchyma cells, fibers, vessels, tracheids and ray cells. Long tubes made up of individual cells are the vessels, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The tracheids ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell. While most conduction in the xylem is up and down, there is some side-to-side or lateral conduction via rays. Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium. In trees, and other woody plants, ray will radiate out from the center of stems and roots and in cross-section will look like the spokes of a wheel.
Phloem
Phloem is an equally important plant tissue as it also is part of the ‘plumbing’ of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls. The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers, parenchyma and ray cells. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Sieve-tube members that are alive contain a polymer called callose. Callose stays in solution as long at the cell contents are under pressure. As a repair mechanism, if an insect injures a cell and the pressure drops, the callose will precipitate. However, the callose and a phloem protein will be moved through the nearest sieve plate where they will for a plug. This prevents further leakage of sieve tube contents and the injury is not necessarily fatal to overall plant turgor pressure
Were stadied different types of plant tissue – meristem, dermal, secretory and ground. Today we shall study VASCULAR and MECHANICAL tissues of plants.
Tissues that support plant are called mechanical tissues. They belong to ground simple tissues, becouse include only one tipe of cells. There are two types of MECHANICAL TISSUES: collenchyma and sclerenchyma. VASCULAR and MECHANICAL tissues of plants contain 90 % of wood in trees.
Collenchyma cells are elongate (up to
Types are classified according to the arrangement of the wall thickenings and include.
angular collenchyma: cell wall is thickest in the corners.
· lamellar collenchyma: cell wall is thickest on two opposite sides.
· lacunar collenchyma: cell wall is thickest in the corners, intercellular air
Collenchyma cells are elongate (up to
Types are classified according to the arrangement of the wall thickenings and include.
angular collenchyma: cell wall is thickest in the corners.
· lamellar collenchyma: cell wall is thickest on two opposite sides.
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lacunar collenchyma: cell wall is thickest in the corners, intercellular air
· Summary: Collenchyma tissues are mainly found under the epidermis
Summary: Collenchyma tissues are mainly found under the epidermis in young stems in the large veins of leaves. The cells are composed of living, elongated cells running parallel to the length of organs that it is found in. Collenchyma cells have thick cellulose cell walls which thickened at the corners or two opposite side/ The cells contain living protoplasm and they sometimes contain chloroplasts. Functions: the collenchyma serve as supporting and strengthening tissue. In collenchyma with chloroplasts photosynthesis takes place.
Sclerenchyma cells are rigid and have thick, nonstretchable secondary cell walls. They support and strengtheonextending regions of plants such as mature stems, and are usually dead at maturity. Sclerenchyma also makes up the hard outer covering of seeds and nuts.
Tissues composed of cells with thick, secondary cell wall that are usually lignified. Types are classified according to cell shape and include:
§ fiber: long, straight and thin, often occurring in bundles. Usually fiber cells are much longer than they are wide and have a very tiny cavity in the center of the cell. Currently, fibers from over 40 different plant families are used in the manufacture of textiles, ropes, string and canvas goods to name a few.
§ sclereids: variable in shape, but not like fibers. Types are classified according to shape and include:
brachysclereids: also called stone cells, length and width nearly equal.
astrosclereids: star shaped, with several projecting arms.
trichosclereids: hair-like, similar to a fibers, except branched.
macrosclereids: column shaped, longer than wide.
osteosclereids: bone shaped, elongated with swollen ends.
VASCULAR TISSUES are specialized for long-distance. Xylem and phloem are the two most important complex tissues in a plant, as their primary functions include the transport of water, ions and soluble food substances throughout the plant. While some complex tissues are produced by apical meristems, most in woody plants are produced by the vascular cambium and is often referenced as vascular tissue.
Other complex tissues include the epidermis and the periderm. The epidermis consists primarily of parenchyma-like cells and forms a protective covering for all plant organs. The epidermis includes specialized cells that allow for the movement of water and gases in and out of the plant, secretory glands, various hairs, cells in which crystals are accumulated and isolated, and other cells that increase absorption in the roots. The periderm is mostly cork cells and therefore forms the outer bark of woody plants.
Vascular tissues derived from the procambium or vascular cambium that transport water and photosynthate. The vascular tissues are complex (composed of several cell types). XYLEM conducts water and dissolved minerals from the roots to all the other parts of the plant. So xylem is the water-conducting tissue of plants. Wood is xylem.
Xylem is a complex tissue composed of xylem vessels (or tracheids), fibres and parenchyma cells. In Angiosperms, most of the water travels in the xylem vessels.
1) Vessel element: a tracheary element with perforation plates. There are vessel element where the secondary cell wall was not deposited and the primary cell wall has been digested. Vessel is a long tube of vessel elements connected by perforation plates. Their diameter may be as large as
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2) Tacheids: a tracheary element that lacks perforations plates, water flows from between tracheids through pits. The xylem of ferns and conifers contains only tracheids.
3) Libriform fiber: a cell in the xylem that is very long and thin and has simple pits, sometimes called “xylary fibers” to distinguish them from extraxylary fibers, which look similar, but have a different evolutionary origin.
4) Xylem parenchyma cells. The thin-walled parenchyma cells have large vacuoles and distinct intercellular spaces.
Xylem is an important plant tissue as it is part of the ‘plumbing’ of a plant. Think of bundles of pipes running along the main axis of stems and roots. It carries water and dissolved substances throughout and consists of a combination of parenchyma cells, fibers, vessels and tracheids. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The tracheids ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell. In trees, and other woody plants, there are ray will radiate out from the center of stems and roots and in cross-section will look like the spokes of a wheel.
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PHLOEM is the photosynthate-conducting tissue of plants. Phloem is a complex tissue composed of sieve elements, companion cells, fiber elements and parenchyma cells. The main components of phloem are sieve elements and companion cells.
1) Sieve element: a conducting cell in the phloem. Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Sieve-tube membe is a sieve element with perforation plates,characteristic of Angiosperms. Sieve tube: a long tube of sieve elements (also called sieve tube members) connected by sieve plates.
2)Companion cells are a cell in the phloem that is connected to a sieve-tube member by numerous plasmodesmata. They have a nucleus.
3) Fiber elements are long and thin (over
4) Phloem parenchyma cells.
Phloem is an equally important plant tissue as it also is part of the ‘plumbing’ of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls. The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers and parenchyma cells.
http://www.youtube.com/watch?v=Yli0FcsQmuI
VASCULAR BUNDLES are classified according to special relationships of xylem and phloem. There are several types of vascular bundles:
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· collateral bundles have xylem on one side and phloem on the other side. There are close and open collateral bundles;
· bicollateral bundles have phloem on both sides of the xylem; they also have cambium.
· concentric bundles which divides to:
o amphicribral – phloem surrounding the xylem;
o amphivasal – xylem surrounding the phloem;
· radial bundles xylem occurs in radial directions, and phloem takes place between them.
All these types are typical for some divisions, classes, families and plant organs. For example:
v closed collateral vascular bundle – for stems and leaves of monocot plants;
v opened collateral vascular bundle – for dicot and gymnosperm plant;
v bicollateral vascular bundle – only for some families of dicot plants (Cucurbitaceae (pumpkin family);
v concentric vascular bundles are common only for rhizomes: with phloem inside – for monocot Angiosperm, with xylem inside – for ferns.
v radial polyarch vascular bundle is typical only for roots of monocot plants, dicot have radial tetraarch or tryarch vascular bundle.
Literature
1. Botany / Randy Moore, W.Denis Clark, Kingsley R.Stern, Darrell Vodopich. – Dubuque, IA, Bogota, Boston, Buenos Aires, Caracas,Chicago, Guilford, CT, London, Madrid, Mexico City, Sydney, Toronto: Wm.C.Brown Publishers.- 1994.-P.44-81.
2. Kindsley R. Stern. Introductory plant biology-
3. Gulko R.M. Explanatory Dictionary of Medicinal Botany- Lviv: LSMU, 2003.-200 p.
4. Laboratory handbook on Pharmaceutical Botany (for students of “pharmacy” specialty) / S.M. Marchyshyn, M.I. Shanayda, I.Z. Kernychna. – Ternopil: TSMU, 2012. – P.7-12.
5. Anatomy of plant сells, tissues, organs and their morphology / Methodical instructions for laboratory works in botany for students of pharmaceutical department / R. Gulko, O. Baran. – L’viv, 2005. – P. 17–25.
6. Botany / [Randy Moore, W.Denis Clark, Kingsley R.Stern, Darrell Vodopich]. –
Prepared by ass. prof. Shanayda M.I.
