plant cell types and structures
TRANSCRIPT
Plant cell types and structures
Parenchyma• It is the most common plant tissue and occurs in most
plant organs as the general ground tissue.
• The cells can vary in form, but young cells are usually isodiametric and thin walled.
• The fundamental parenchyma present in various parts of the plant is potentially meristematic, and such cells achieve maturity without differentiation except for an increase in cell size and wall thickness and a restricted change of form.
parenchyma
• Meristematic tissues are composed of cells, which are in a state of division, or capable of undergoing division, if and when needed.
• By maturity changes in shape and appearance of intercellular spaces may occur to give more distinctive type of parenchyma tissue. E.g. pitted parenchyma in Lobelia inflata. When a parenchymatous tissue contain chloroplast it is called chlorenchyma whose function is to manufacture food material.
Functions of Parenchyma cells
• Storage– flesh of fruit– storage in roots and seeds
• basic metabolism
Parenchymatous cells
Collenchyma• Is the typical supporting tissue of young herbaceous stems and
leaf midribs (petioles). • It is similar to parenchyma except that the primary cell wall is
thickened to give greater mechanical strength. The cells are much thickened at the corners due to the deposition of cellulose impregnated with pectin.
• The tissues are composed of elongated cells with oblique, rounded or tapering ends. They appear spherical, oval, or polygonal in transverse section. Intercellular spaces may or may not occur. It is absent from the roots. Being flexible in nature, collenchyma tissue gives tensile strength to the growing organs.
Collenhyma cells
Thickened corner of cell wall
Sclerenchyma• Sclerenchyma: Is a hard supporting tissue with
heavy secondary thickening.
• Sclerenchymatous cells are usually divided into two categories according to their aspect ratio1. Fibres and 2. Sclereids (stone cells)
sclerenchyma
Sclereids (stone cells)• Sclereids (stone cells) are typically roughly isodiametric, although
elongated and branched forms often occur.
• The walls of the typical sclereid are thick, lignified, often showing well-marked stratification (and traversed by pit canals which are funnel shaped or branched).
• Stone cells commonly occur in the hard outer coats of seeds and fruits and in the bark and pericyclic regions of the woody stems. They may be found singly, or in small groups as in quillaia or in larger groups in cascara bark, or as a complete layer in cinnamon and cassia barks.
• The absence of sclereids from frangula and cinchona aids in their microscopical identification
Stone cells
Fibres• Fibres are typified by a high length to width ratio. They
are usually thick walled and have a narrow lumen and pointed ends.
• Fibres are usually classified according to the area in which they occur as pericyclic, xylem, or phloem fibres.
• A crystal sheath is sometimes formed around sclerenchyma and this feature, together with the size, frequency and distribution of the cells is often of diagnostic importance.
Fibers are used for clothing
• Cotton• Linen
Fibres
Epidermis• Epidermis: This is the outermost layer of the plant
structure and consists of a single layer of cells covering the whole plant. However, the outermost layer of the root is known as the piliferous layer.
• They make up the dermal tissues of the plant
• Epidermal cells show great variety in form, giving characteristic patterns when seen in surface view.
• In transection they are often flattened parallel to the surface, and square or rectangular in shape.
• These are of diagnostic importance. They may be straight-walled and polygonal as in senna leaf; wavy-walled e.g. hyoscyamus leaf, stramonium and belladonna; beaded walls e.g. Digitalis lanata and lobelia inflata ; a papillose epidermis in coca leaf etc.
• A = straight-walled polygonal (senna leaf)
• B = wavy walled (hyoscyamus leaf)
• C = beaded (D. lanata)• D = slightly wavy with
striated cuticle• E = papillose
(pyrethrum)
Epidermal cell structures
• Distributed amongst the relatively non-specialised epidermal cells are a number of highly specialised and characteristic structures.
• The most universal of these is the stomata.
• Trichomes may also be present in the epidermis. They occur most frequently on young leaves and stems, but are also found on other organs such as flowers.
stomata• Stomata: are specialised epidermal structures for the gaseous
exchange and the control of water loss from the plant.
• They are commonly observed in leaves.
• A stoma consists of a central pore surrounded by two guard cells. It may be flush with, raised above, or embedded below the surrounding epidermal cells.
• The arrangement of the epidermal cells around the stoma falls into one of several main types, which helps in the identification of the stomata and the plant.
stomata
Types of stomatal arrangements
• Anomocytic stomata have no particular arrangement of the epidermal cells
• Anisocytic have three or four subsidiary cells with one being smaller than the others.
• Paracytic have two subsidiary cells with their long axes parallel to the pore
Types of stomatal arrangements• Diacytic have two subsidiary cells with their long
axes perpendicular or at right angle to the pore of the stomata.
• Actinocytic (radiate-celled) where the stoma is surrounded by a number of radiating subsidiary cells arranged along the radii of a circle. This type of stomata is not very common in drug plants.
• Graminaceous -Dumbell shaped guard cells surrounded by elongated epidermal cells
• The distribution of stomata between the upper and lower epidermis shows great variation. The stomata may be entirely confined to the lower epidermis as in e.g. Ficus species, They may also be confined to the upper epidermis as in some aquatic plants or sometimes they may be evenly distributed on both surfaces.
Trichomes• Trichomes-:are elongated tubular outgrowths on the
epidermal cells of leaves, many herbaceous stems, flowers, fruits and seeds.
• They are also called epidermal hairs. The occurrence and forms of the trichomes are very valuable characters for the identification of leaf drugs and detection of adulterants in them.
• They may be absent from many important leaf drugs like Coca and hemlock but many leaves possess them in abundance.
Two main types of trichomes are normally recognised
• a) covering trichomes and
• b) glandular trichomes
• Any one type or more than one type of trichomes may be found on the same plant
Covering Trichomes
• They may be unicellular or multicellular. The unicellular vary greatly for example short conical trichomes are present in Tea leaves; short conical and warty trichomes in Senna; large conical and longitudinal striated trichomes in Lobelia.
• The multicellular trichomes may be uni, bi or multiseriate or even branched. The number of cells in the multicellular trichomes may vary from two to many cells.
Glandular trichomes• Glandular trichomes: They usually have a uni or
multicellular stalk and uni- or multicellular head.
• The stalk may be uni- or multiseriate and short or long.
• The glandular trichomes may also be sessile embedded in the epidermal cells.
• The type of the glandular trichomes may be characteristic of a plant family or genus for e.g. glandular trichomes with short unicellular stalk and multicellular rounded or oval head are found in the Solanacaeae.
Glandular trichomes
• A = bicellular head and unicellular stalk
• B = uniseriate with unicellular head
• C = uniseriate collapsed and twisted with slightly warty wall
• D = unicellular stalk and multicellular head
• E = multiseriate stalk and multicellular head (cannabis)
• F = unicellular thick warty walled covering
• G = multicellular stellate
• A = bicellular head and unicellular stalk• B = uniseriate with unicellular head• C = uniseriate collapsed and twisted with slightly
warty wall• D = unicellular stalk and multicellular head• E = multiseriate stalk and multicellular head
(cannabis)• F = unicellular thick warty walled covering • G = multicellular stellate
Functions of covering trichomes• Covering trichomes function as
• protective agents,
• light screen for reducing rate of transpiration, • climbing agent,
• water storing agent and also
• as seed dispersal agent
Functions of glandular trichomes
• Mainly as a storage point for
• the secretion of poisonous substances,
• volatile oils, resins,
• mucilages etc.
Cuticle• The Cuticle:- The outer wall of epidermal cells
are often covered with a water-proof transparent layer of cutin, which is called the cuticle.
• The cuticle is a protective layer and is usually very thin but with excess deposition of pro-cutin it may have ridges and become somewhat striated
Periderm• Periderm : Is a protective tissue which replaces the epidermis in
stems and roots which have continual secondary growth.
• It is formed from the cork cambium (phellogen) which produces cork (phellem) on the outside and secondary cortex (phelloderm) on the inside.
• The presence or absence of cork or secondary cortex is important in those drugs which are prepared by removing certain outer layers from a bark, e.g. peeled ginger, cinnamon.
• Cork cell are suberised (waterproof) and non-living at maturity, may be lignified, and are usually yellowish or brown. Stratification sometimes occurs.
Types of cork
Cortex• Cortex: This zone consists of a few to many layers of cells and lies
between the epidermis and the pericyle.
• In a dicotyledonous stem the cortex is differentiated into: • (a) hypodermis, which consists of a few layers of collenchyma or
sometimes sclerenchyma cells;
• (b) general cortex, which is made up of a few layers of thin-walled parenchymatous cells with or without chloroplasts, often with intercellular spaces, and
• (c) endodermis is a specialised layer of cells marking the inner cortex.
Root X’section
Endodermis• The endodermis is a specialised layer of cells forming the inner cortex
• The endodermal cells are living containing protoplasm, large nuclei, and often starch grains.
• When starch grains are present, the endodermis is also called starch sheath.
• The outer walls of the cells are thin, while the radial and inner walls are often thickened being suberised or cutinised.
• A typical endodermis is found typically in roots and rhizomes and in certain stems. Usually present in roots, in aquatic and subterranean stems and the aerial stems of certain families e.g. Labiatae and Curcubitaceace.
• Endodermis is absent in woody plants and leaves of higher plants.
Tissue distribution of rhizome and root of a typical dicot
Vascular cylinder
Pericycle• The pericycle is a multi-layered zone between the
endodermis and the vascular bundles.
• It is sometimes composed of only sclerenchymatous cells, but most often a mixture of both parenchyma and sclerenchyma cells constitute the pericyle.
• It is absent in the roots and stems of some aquatic plants and is not distinguishable in the stems of monocotyledonous plants. Single-layered pericycles also occur in the roots of higher plants.
Pith or medulla• The central soft part of the stem and root is called
pith or medulla. It is made up of large parenchymatous cells with intercellular spaces.
• The pith is large and well developed in the stem of dicotyledonous plants
• while it is indistinguishable in those of the monocotyledonous plants.
Pith or medulla• It is small or absent in the dicotyledonous roots, while in the
monocotyledonous root a distinct pith is present.
• In the stems of the dicotyledonous plants the pith or medulla extends outwards to the pericycle between the vascular bundles.
• These extensions of the pith are called pith rays or medullary rays. These are not present as such in the root.
• Based on the occurrence of the pith and pith ray, therefore, the major groups of some herbaceous and root drugs may be identified.
CORK TISSUE• As the plant axis increases in diameter, a cork cambium or
phellogen usually arises which by its activity produces new protective tissues, known collectively as periderm. Which replace the epidermis and part of the entire primary cortex.
• The cells of the cork cambium undergo tangential divisions giving rise externally to phellem or cork tissue and internally to phelloderm or secondary cortex
CORK TISSUE• Usually only a limited production of phelloderm occurs. So that the
number of cork layers greatly exceeds the number of phelloderm layers.
• The stele is the central region of a stem or root surrounded by the endodermis and consists of the pericycle, vascular bundles, pith and medullary rays.
Vascular bundle
• Each vascular bundle is composed of xylem and phloem with or without a cambium between them or simply of either xylem or phloem.
Vascular bundles in dicot
Vascular bundles in dicot
Xylem• The primary xylem is composed of protoxylem and
metaxylem.
• Secondary growth in thickness of the stem and root of gymnosperms and dicot is accompanied by the formation of secondary xylem
Xylem• is the principal water conducting tissue of a plant. • It is a compound tissue made of parenchyma,
fibres, tracheids and vessels {treaheary elements). • The treaheary elements are responsible for
conducting water from the root to the various plant parts. They have lignified secondarily thickened walls which can be in a variety of forms.
Xylem• The essential difference between tracheids and
vessel members is that the former are imperforate whereas the vessels have pores at each end and are connected to form a continuous file or tube.
• In both tracheids and vessels the wall thickening is usually laid down in a particular manner to give a pitted, annular, reticulate, scalariform or spiral pattern.
vessels
• A = annular, B = spiral, C = scalariform• D = reticulate, E = pitted, F = bordered
xylem
• The tracheid is derived from a single cell and can be regarded as the cell type of the xylem tissue.
• It takes the form of an elongated, water conducting cell with a lignified and variously thickened and pitted cell wall. Pits may be simple or bordered.
A cross-section through xylem tissue
A longitudinal view of xylary elements
PHLOEM• Phloem is also a compound tissue and it is
responsible for the transport of food.
• It contains parenchyma, sclerenchyma, secretory cells and sieve elements (sieve tubes and companion cells).
Phloem• The sieve elements are the most highly specialised cells and
their chief morphological character is the occurrence of sieve areas (modified pits) in their wall.
• The sieve tube is the conducting element of the phloem. Phloem sclereids are important in the identification of certain barks.
• The sieve elements are usually broken during powdering of the plant.
A cross-section through typical phloem tissue
Monocot vascular bundle
Arrangement of vascular bundles• Types of Vascular bundles The bundles may be
regularly arranged in a ring, as in the stems of dicotyledonous plants and roots of all plants or they may be scattered in the ground tissue, as in the stems of monocotyledonous plants.
• On the basis of the arrangement of xylem and phloem and the presence or absence of cambium in the bundle, the following two main types of vascular bundles may be recognised, each of them being characteristics of certain plant organs: (1) Radial and (2) Conjoint.
• Radial: In this type, xylem and phloem form separate bundles which occur on different radii alternating with each other. Radial vascular bundles are characteristic of the roots, e.g., Rauwolfia root.
• Conjoint: Here both xylem and phloem occur together forming one compact vascular bundle. This type of bundles is characteristic of all stems, monocot or dicot. Depending on the relative position and arrangement of the two component tissues, the following three types of conjoint vascular bundles can be differentiated:
Conjoint• a) Collateral, when xylem and phloem lie together on the same
radius, xylem being internal and phloem external. If a cambium is present between the xylem and phloem tissues, the bundle is called an open collateral bundle, and if no cambium is present between them, it is said to be a closed collateral bundle. Open collateral bundles occur in the stems of dicotyledonous plants, whereas closed ones are found in those of monocotyledonous plants.
• b) Bicollateral, when in a collateral bundle both phloem and cambium occur twice, once on the outer side of the xylem and then again on its inner side. The various tissues of a bicollateral vascular bundle occur in the following sequence: outer phloem, outer cambium, xylem, inner cambium and inner phloem. This type of bundles is often found in many drug plants of Solanaceae, Apocynaceae, Myrtaceae and Cucurbitaceae.
• c) Concentric.
• 1 upper epidermis3 spongy parenchyma4 air cavity5 lower epidermis7 trichome8 major vein9 xylem2 palisade parenchyma6 stomata10 phloem11 supporting tissue (sclerenchyma)
• English name: LilacScientific name: Syringa vulgarisFamilia: OleaceaeClassis: DicotyledonasPhylum: Angiospermae
Monocot stem X’section
Secretory tissues• These are important features of certain drugs whose
therapeutic constituents are contained in the secretions.
• In addition they are useful diagnostic features in other cases.
• They include secretory cells, secretory cavities or sacs, secretory ducts or canals and latex tissue. The secretions are variable and cells may contain oil, mucilage, crystals tannins resin or latex.
Secretory cells
• Secretory cells may produce oils e.g. ginger, mace, cinnamon and cassia.
• Large oil cells form an important diagnostic character of powdered sassafras root bark. Cells containing resins, oleoresins and mucilage are common.
Secretory cavities or sacs• May arise by separation of the cells and subsequent formation of a
secretory epithelium (schizogenously) or by breakdown of the cells forming a cavity not bounded by a definite epithelium (lysigenously). Schizogenous oil cavities occur in eucalyptus, lysigenous cavities in Gossypium species.
• Secretory products may appear in cells before the latter break down to give a lysigenous cavity. Schizolysigenous oil cavities occur in the Rutaceae and Burseraceace.
• The oil cavity develops from a mother cell, which undergoes division to give daughter cells which separate, leaving a schizogenous central cavity. The walls of the cells surrounding this central cavity then break down, forming an oily secretion, and the cavity continues to increase in size lysigenously.
Secretory ducts or canals• The vittae of the umbelliferae are schizogenous
oleoresin canals and they occur in the stem, roots and leaves.
• The oleoresin ducts of Pinus species are also of schizogenous origin. Schizogenous oleoresins ducts, which enlarge lysigenously, occur in some members of the leguminosae e.g. Copaifera.
Latex (laticiferous) tissue• Consists of either cells or tubes which contain a fluid with a
milky appearance arising from the suspension of small particles in a liquid dispersion medium with a very different refractive index.
• The suspended particles vary in nature, and may be hydrocarbons composed of essential oils, resins and rubber.
• Alkaloids are present in the latex of Papaveraceae, the proteolytic enzyme papain in the latex of Carica (pawpaw) and vitamin B1 Latex cells are typical of the families in Euphorbiaceae, Moraceae, Cannabinaceae.
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