In this article we will discuss about the plant cell which is the fundamental unit of all living organisms in terms of structure and function. This will also help you to draw the structure and diagram of plant cell.
History of Discovery:
Robert Hooke in 1665 first discovered plant cell. The term ‘cell’ was coined to describe the small walled units that were observed in the sections of bottle cork under simple microscope. Between 1831-’33 Robert Brown described the nucleus as small spherical body, which is normally present in every plant cell.
In 1864 Mohl gave the name protoplasm to designate the thin mucilaginous layer present inside the cell wall. Shimper and Meyer in 1880 discovered plastids. Thus by more than hundred years ago the largest structural elements of a plant cell like cell wall, nucleus, protoplasm, plastid etc. were described.
In later years especially after 1950s with the introduction of electron microscope like SEM, TEM, improved techniques like enzyme cytochemistry, autoradiography, x-ray detraction study, biochemistry etc. the structural and functional elements of a plant cell have been revealed.
Structure of Plant Cell:
Two main regions can be recognized in a plant cell, i.e. the cell wall and protoplasm. Protoplasm consists of nucleus and cytoplasm, and the latter contains a variety of organelles, vacuole, salts and various organic molecules.
The various components of a plant cell is outlined in the following table:
The details of cytoplasmic organelles and reserve ergastic substances are fully described in the textbooks of Cytology and Biochemistry and will not be discussed in details here.
Each plant cell is surrounded by a carbohydrate rich rigid wall termed cell wall that distinguishes them from animal cell. (The reproductive cells have no walls). It limits and controls cell growth, binds with neighbouring cells to form tissue, forms a protective barrier against infection and lends skeletal support to the whole plant. (The cell wall provides a source of food, fibre and fuel for man).
The main component of cell wall is cellulose fibrils embedded in an amorphous matrix composed of polysaccharides and proteinaceous materials. The architecture of cell wall is based on cellulose, which forms the framework of wall and the other components like pectin, lignin, chitin, suberin etc. inlay the cellulose framework.
The amicroscopic cellulose molecules (8Å width) are combined into an elementary microfibril (100Å width) that forms a bundle called microfibril (250Å thick and contains 2000 cellulose molecules). Electron microscopic study revealed that the units of microfibril aggregated into coarser fibril —termed macrofibril.
The substance present within cell wall including plasma membrane is generally called protoplasm. Normally this term is used to describe the portions left of a plant cell after the removal of cell wall. It is colloidal in nature and consists of nucleus and cytoplasm.
Robert Brown in 1831 first described plant cell nucleus from the stamen filaments of Tradescantia. It is universally present in all cells except mature sieve tubes and mammalian erythrocytes where nuclei are present at the early stages of development. It is the largest cellular organelle of about 10 µm in diameter although considerable variations exist in size between cells of different species.
In meristematic cells nucleus is roughly spherical in shape and forms 75% of cell volume. The shape may be flattened in differentiated cells where a central large vacuole is present and the nucleus lies against the cell wall. Cylindrical nucleus is seen in elongated cells.
The nucleus consists of a number of components:
(a) Nuclear envelope or membrane,
(b) Nuclear sap or karyolymph,
(c) Nucleolus and
(d) Nuclear reticulum.
(a) Nuclear envelope or membrane:
The nuclear contents and nuclear sap remain enclosed by nuclear envelope. This membrane separates the nuclear sap with its contents from cytoplasm. Electron microscopic study reveals that this envelope consists of two unit membranes separated by a gap. This gap is known as perinuclear space.
There are many pores on the membrane and the pores may occupy 8% area of the membrane (Fig. 1.2). The inner and outer pore margins show distinct eight globular sub-units called annular granules. Granule may be present at centre also, which suggests that it regulates the free communication between nucleus and cytoplasm.
(b) Nuclear sap or karyolymph:
At interphase the nucleus contains granular and homogeneous fluid that is karyolymph. The fluid contains chromatin materials (Euchromatin and Heterochromatin), nucleoli and protein materials some of which are in the form of nuclear enzymes (nuclear protoplasm).
It is roughly spherical in shape and is present in every non-dividing cell. It is not bounded by a membrane and can be clearly seen even in light microscope. In dividing cells nucleolus disappears and its reappearance is due to the activity of a special region of certain chromosome known as nucleolar organizer.
Though more than one nucleolar organizer may be present the developing nucleoli fuse to form a single nucleolus at maturity.
Electron microscopic study shows distinct regions within the nucleolus:
(i) A peripheral granular region that is thought to be the precursors of cytoplasmic ribosome,
(ii) Fibres composed of ribonucleoprotein and
(d) Nuclear reticulum:
The un-dividing nucleus shows a reticulate structure made up of chromosomes and are bounded by nuclear membrane and remains embedded in karyolymph. During the process of nuclear division or mitosis this chromatin reticulum is organized to chromosomes, which are the bearer of the hereditary characters and ‘the biological unit of heredity control’ the genes are located in them.
Each plant species possesses constant number of chromosomes. DNA, RNA and various proteins compose a chromosome. Electron microscopic study revealed the globular repeating units in a chromatin—termed nucleosome.
Function of Nucleus:
As the nucleus is the major site of DNA, it is responsible for the replication, expression and preservation of genetic information. It also regulates cell division, growth and differentiation. The hereditary characters are transmitted through nucleus.
It is the portion of protoplasm that is inside the plasma membrane and outside the nucleus. It is a heterogeneous mass containing granules of different sizes and shapes. The mass of cytoplasm contains many organelles and aqueous substance in which many inorganic or organic particles remain dissolved or suspended.
The aqueous portion of cytoplasm is termed as hyaloplasm where the organelles are embedded. In the differentiated cells cytoplasm possesses one or more vacuoles that contain cell sap. The cell sap is composed of different salts, sugars and in some cases the specific pigment anthocyanin. The cytoplasmic organelles include ribosome, mitochondria, plastids, microbodies, Golgibodies, lysosomes and endoplasmic reticulum.
Robinson and Brown in 1953 first noted ribosome in bean roots. They occur both free in the cytoplasm and attached to endoplasmic reticulum. They are also present in the chloroplastids. In many cells a thin thread joins several ribosome. These aggregates of ribosome are known as polyribosome or polysome.
The cytoplasmic ribosomes are composed of two subunits and Mg++ ions bind them together with a sedimentation constant of about 80S. When Mg++ is removed the 80S particles dissociates into 60S and 40S subunit. The ‘S’ values are not additive because the shape and size of the particle may change the rate of sedimentation. They may associate again when Mg++ is restored.
Ribosome consists of approximately equal parts of protein and RNA. The ribosome is the site of protein synthesis. The mitochondrial and chloroplast ribosome also incorporate amino acids into protein. Sometimes ribosome of organelles and cytoplasm function in a coordinated way.
The enzyme protein Ribulose biphosphate carboxylase is synthesized within chloroplast and consists of large and small subunits. The small subunit is synthesized at the cytoplasmic ribosome and then it moves into chloroplast to form Ribulose biphosphate carboxylase.
Altman in 1894 first observed these organelles and called them ‘bioplasts’. Benda in 1897 called them mitochondria (Greek: mitos = thread, chondrion = granule). Their occurrence in plant cells (ex. Nymphaea) was first reported by Meves in 1904.
They are just visible as small rods or spheres under light microscope. They can be observed easily in a phase contrast microscope under dark field illumination when stained in Janus green B. In later years they are isolated by differential centrifugation, ultracentrifugation etc. and are widely studied with electron microscope (Fig. 1.3).
The shape of mitochondria is not constant and this variation occurs during certain functional stages. They may be filamentous, granular, or they may assume the shape of a club, a tennis racket or vesicular. The shape is more or less constant in similar type of cells that perform same function.
Mitochondria are present in all cells that respire aerobically. Their number varies from cell types. The number may be as high as 5000,000 in the protozoon Chaos chaos. In a particular cell type the number is more or less constant and the plant cells contain lesser number of mitochondria than animal cells.
In general, mitochondria are distributed uniformly throughout the cytoplasm. They may however accumulate preferentially at the proximal, basal, peripheral side of a cell or around the nucleus. During mitosis mitochondria aggregate near the spindle and at the end of division they are distributed in the daughter cells in approximately equal number.
The size of mitochondria is variable. The width is about 0.5 µm and it is relatively constant in most cells. However the length may be as high as 7 µm. Electron microscopic study reveals that mitochondrion consists of two membranes — an outer and an inner membrane.
The outer membrane is smooth, about 6 nm thick and it is the limiting membrane that surrounds each mitochondrion. The inner membrane may form plate like folds called mitochondrial crests (cristae) or tubular projections termed microvilli that penetrate the matrix of mitochondrion. The inner membrane is also about 6nm thick.
The arrangement of cristae varies. They may be parallel or perpendicular to the long axis of mitochondrion. They may be in the form of vesicles or branched to form a network. Sometimes they are arranged concentrically within the matrix. The underlying purpose of the enfolding is to provide an increased surface area within the mitochondrion for enzymatic activity.
This inner membrane divides the mitochondrion into two chambers:
(i) Inner chamber or inner membrane space that is bounded by the inner membrane and
(ii) Outer chamber or intermembrane space that is represented by the intermediate space between the two membranes and core of the mitochondrial crests.
The outer chamber is filled with watery fluid and is 140 Å to 180 Å in width. The inner chamber is filled with dense proteinaceous matrix. The matrix contains granules of phospholipids, ribosomes, which are slightly smaller than those in the cytoplasm and DNA (mitochondrial DNA). The microvilli and cristae are, in general, incomplete septa and so there is continuity of the matrix.
On the inner side of inner membrane, i.e. the side facing the matrix, there are attached ‘elementary’ or ‘F1‘ particles or oxysomes of 8.5 nm in diameter. These particles are shortly stalked and remain attached to the inner membrane at regular intervals of 10nm. There may be 104-105 numbers of elementary particles per mitochondrion.
Mitochondria are regarded as the ‘power house of a cell’ and they are concerned with energy conversion. Several enzymes of respiratory cycle and all the enzymes of Krebs cycle are found in mitochondria.
Electron transport system, oxidative phosphorylation and ATP synthesis occur here. Mitochondria may move and carry ATP wherever required. The movement is less in animals than plants. Lipid synthesis and elongation of fatty acids occur in mitochondria.
Mitochondria are semiautonomous organelles and they can synthesize protein due to the presence of DNA, ribosome and RNA in the matrix.
Plastids are small, variously shaped subcellular organelles present in cytoplasm of plant cells. Their number varies in different plants. They develop from pro-plastids.
They are classified on the basis of colour and function, e.g. chromoplast (non-green and non-photosynthetic pigment plastid), leucoplast (colourless plastid), amyloplast (starch accumulating plastid), proteinoplast (protein storing plastid), elaioplast (fat accumulating plastid) and the very prominent chloroplast (green, contains chlorophyll and other photosynthetic pigments).
Chromoplast contains carotenoid and the colour may be red, orange or yellow. They are generally produced from amyloplasts or chloroplasts. Amyloplast synthesizes starch and stores one or more starch grains. The protein containing plastids proteoplast (or proteinoplast) are devoid of grana and contain few thylakoids. Elaioplasts, in some species, are produced from chloroplastid.
Chloroplastid consists of a double membrane enclosing homogeneous stroma and grana. Granum remains embedded in stroma and consists of flattened, discshaped vesicles forming lamellae known as thylakoids (Fig. 1.4).
They are spherical or oval cytoplasmic organelles but usually appear circular in cross section. They are found closely associated with endoplasmic reticulum: Biochemical studies distinguish two types of microbodies — peroxisomes and glyoxysomes. They differ in their enzyme content.
Peroxisomes are present in the leaves of higher plants and they function in close co-operation with chloroplastids. They are the sites of photorespiration. Glyoxysomes are abundant in germinating fatty seeds and they contain enzymes for the breakdown of fatty acid.
Golgi Body or Dictyosome:
Golgi body or dictyosome (as it is frequently called in plant cells) is composed of stacks of smooth membranes (Fig. 1.5) and disc shaped sacks called cisternae. Camillo Golgi first described these in 1898.
They are usually found scattered throughout the cytoplasm and their number varies from one to several hundred per cell. In sections cisternae appear like flattened sacs and their number may be as high as twenty in each Golgi body.
Some cisternae are dilated towards the periphery and appear as vesicles. The outer edges of cisternae consist of network of tubules. The major function of Golgi complex is secretion. Other functions are acrosome formation during sperm maturation, release of zymogen granules, storage of protein or lipid in vesicles and vacuoles, formation of plasma membrane, polysaccharide formation for the new cell wall and primary lysosome formation.
They are sub-microscopic and single membrane bounded particle. They lack any internal structure and contain hydrolytic enzymes. They can be observed under electron microscope only. They are the main sites of hydrolytic enzymes and so can hydrolyze protein and carbohydrate. They originate from Golgi complex.
They are also called oleosomes. These are small spherical organelles of 1 µm in diameter. They are surrounded by a half-unit membrane rather than a complete unit membrane. They consist of lipid droplets. They also contain a variety of enzymes namely phosphatases, lipases, RNAase and DNAase, endopeptidases etc.
They originate as oil containing vesicles from the endoplasmic reticulum. They are one of the sites of fat and oil synthesis. They store fats. The number and frequency of occurrence of spherosomes vary in different cells. Large number of spherosomes is reported from the guard cells of Campanula persicifolia, Paphiopedilum etc.
The cytoplasm of a cell permeated by complex elaborate system of membranes termed endoplasmic reticulum (ER). Electron microscopic study discovered the existence of ER in 1945 by Porter et al. and the term ER was used by Porter and Kallman in 1952. Now a days ER can be observed in a living cell by the use of fluorescent dyes.
ER consists of double membranes that enclose spaces. They may be in the form of tubules, vesicles, vacuoles and flattened sacs (cisternae) that at several points are intercommunicating. When the tubules were first observed, the micrographs showed that the tubules did not reach the periphery of the cell and hence the term ‘endoplasmic’ was coined.
The diameter of the tubule may be 50 µm to 100 µm. The vesicles are more or less round in shape and range in diameter from 25 µm to 500 µm. The cisternae are long flattened units, 40 |nm to 50 µm thick. They are arranged in parallel stacks (Fig. 1.6).
Endoplasmic reticulum exists in two forms — smooth and rough; the two forms may be interconnected with each other. The rough form is due to the presence of numerous small particles of ribonucleoprotein on the surface of ER.
The functions of ER are suggested as:
(1) The membrane provides increased surface area for metabolic activities.
(2) The spaces formed by ER are the site of collection of metabolic products.
(3) The rough ER are the site of protein synthesis.
(4) The smooth ER are present in those cells that synthesize cholesterol, glycerides etc.
(5) ER forms an effective transport system for protein etc.
(6) It has role in the formation of cell wall.
(7) It transmits impulse.
The nonliving cell inclusions produced as a result of metabolic activity of cell are known as ergastic substances. These substances are found in cytoplasm, vacuoles and cell walls. Some of these have great importance in plant life while others are absolutely by-products.
These substances are grouped as:
(1) Reserve material,
(2) Secretory material and
(3) Excretory material.
1. Reserve material:
The autotropic plants by photosynthesis synthesize carbohydrate that along with other metabolite forms proteins, fats and oils. These substances may be used up to make new protoplasm and to provide energy for cellular activities or may be stored as reserve food materials for future use. The reserve materials are carbohydrate, protein, oil and fats.
Carbohydrates (= carbon hydrates) are composed of carbon, hydrogen and oxygen where the ratio between the latter two is two: one. They are often referred to as saccharides (Latin, Sacchariim = sugar) because the simpler members of the carbohydrate taste sweet.
The elements of carbohydrate are obtained from atmospheric carbon dioxide and water during photosynthesis. Carbohydrates are classified into monosaccharides, oligosaccharides and polysaccharides on the basis of number of simple sugar molecule produced on hydrolysis.
The product of hydrolysis may be of same or different sugars:
i. Monosaccharide (simple sugar):
These are composed of single unit of carbohydrate. They cannot be broken into simpler carbohydrates on hydrolysis. Example: glucose (present in grape sugar), fructose (found in honey) etc. They are also referred to as hexose as six carbon atoms are present in these sugars.
They contain two to ten units of monosaccharide in a molecule. The oligosaccharides containing two units of monosaccharide are called disaccharide (ex. sucrose); those containing three units are called trisaccharide (ex. raffinose).
Sucrose of cane sugar yields glucose and fructose, one molecule each on hydrolysis. It is the table sugar and is extracted from sugarcane. Another disaccharide — maltose yields two molecules of glucose on hydrolysis. The trisaccharide raffinose yields glucose, fructose and galactose on hydrolysis.
Each molecule of polysaccharide is made up of more than ten units of monosaccharide. These include the storage polysaccharide and the structural polysaccharide inulin, starch, cellulose, pectin etc.
This polysaccharide yields only fructose on hydrolysis. It is found as a storage material in some members of Asteraceae (tuber of Dahlia, Helianthus tuberosus), Campanulaceae and monocotyledons. Inulin is spherical or star shaped crystal. Incompletely formed crystals can be seen as fan-shaped structure.
It appears only in cell sap solution of storage organs. They may be precipitated when thin pieces of Dahlia are treated with glycerine or alcohol for about a week. A thin transverse section of the treated tubers will reveal the spherical or star shaped or fan shaped crystals (Fig. 1.7).
The glucose units are the building blocks of this insoluble polysaccharide. They are found in all parenchymatous tissues mainly in storage organs like rhizomes, endosperm, corms, tubers, cotyledon of seeds etc. They are also present in latex tubes of stems and roots, endodermis, seeds of cereal, grains of rice, wheat, maize etc.
The assimilatory starch, a temporary product is synthesized by the chloroplasts. This temporary starch is subsequently broken down to sugar that is transported to the storage organs where storage starch is synthesized by amyloplast.
Starch appears in the form of grains that stain bluish black with potassium iodide solution. Each starch grain consists of a centre, called hilum that is the centre of origin of the grain. Carbohydrates are deposited in layers around the hilum. This gives starch grain a striated appearance. The deposition of layers depends upon endogenous rhythms.
In cereals, single layer is deposited in a day, i.e. the number of striations denotes the number of days’ growth. The hilum may be centrally situated as in concentric grains where carbohydrates are laid down concentrically around the hilum (e.g. Triticum durum) or the hilum may be eccentric as in case of eccentric grains where the layers of carbohydrates are deposited on one side of hilum (ex. Solarium tuberosum) Fig. 1.8.
Starch grains may be classified as simple, semi-compound or half- compound and compound. In simple type each grain remains singly with solitary hilum whereas in compound type two or more grains are aggregated together (ex. Oryza sativa, Ipomoea batatas, Avena sativa) with their separate hila.
The half compound grains (e.g. potato) consist of two hila with their lines of stratification aggregated together around which several common layers of carbohydrate are present.
Starch grains vary in shapes, sizes, position of hilum, solitary or in aggregates and in general appearance. Their extensive morphological variations make it possible to identify starch containing plant species.
This insoluble structural polysaccharide is the major constituent of cell wall of higher plants. The building blocks of cellulose are glucose units that are held together by β (1-4) linkages. In cellulose molecule approximately 15,000 molecules of glucose residues are present.
This structural polysaccharide is chiefly present in the middle lamella and primary walls of dicots. Pectins are rich in galacturonic acid, rhamnose, arabinose and galactose.
Proteins are composed of amino acids that are linked by peptide bonds. They are one of the vital ingredients of living protoplasmic bodies. They sometimes occur as structural proteins of cell wall and storage proteins. Storage protein may be in the form of definitive bodies called aleurone grains or they may be amorphous or crystalline.
Amorphous protein is found in the aleurone layer (the outermost layer of endosperm) of Triticum. Crystalline protein possesses both crystalline and colloidal properties. So these crystalline proteins are referred to as crystalloids. Proteins, in the form of cuboidal crystalloids are observed in the peripheral parenchyma cells of potato tuber and in the fruit of Capsicum.
The reserve proteins of potato are mainly composed of lysine and it is removed when the potatoes are peeled. Aleurone grains (Fig. 1.9) are smaller in contrast to starch grains. They stain brown with potassium iodide solution. They are found in the cells of seeds, embryos, endosperm etc. They may occur in specific layers as is noted in the aleurone layers of caryopsis of cereals.
Aleurone grains are bounded by a proteinaceous membrane. The membrane may enclose amorphous protein, globoids, and crystalloids (e.g. Myristica fragrans) or crystals of calcium oxalate (Umbelliferae). The aleurone grains of Ricinus usually contain one globoid and one crystalloid, bounded by a single membrane.
(C) Oil and fat:
They are the glycerides of fatty acid. The oils are liquid and the fats are solid at normal temperature. In contrast to protein and carbohydrate they provide more calories and so they are valuable reserve food materials. They are abundant in the cells of endosperm or perisperm of seed, fruits, spores, embryos, and meristematic cells and in differentiated tissues of vegetative body.
They occur also in the cytoplasm of cell sap as dispersed or aggregated large masses. They are produced either by elaioplast or spherosomes. The essential or volatile oils may occur in all tissues (e.g. Conifers), in petals of roses, in the fruit skin of orange, in the special layer (i.e. second layer below the epidermis) of Elettaria cardamomum seed.
Specialized secretory tissues produce these essential oils. Wax, cutin and suberin are other fatty substances, which usually form a protective covering of the epidermal cell walls. Oils and fats stain a reddish colour when treated with Sudan III or IV.
2. Secretory material:
These substances, though not concerned with nutrition are valuable ergastic substances. They are produced during plant metabolism and perform specific roles in plant life.
(i) Colouring matter,
(ii) Enzyme and
i. Colouring matter:
Leaves, petals and fruits of a plant exhibit various colours. This is due to the pigment present in the cell. The leaves and aerial organs possess chlorophyll that imparts green colours to them. This pigment is responsible for the phenomenon-photosynthesis. The other pigment carotenoids, i.e. carotene and xanthophyll impart orange to yellow colour of many flowers and fruits.
The red, blue, violet and pink colour is due to anthocyanin-the water-soluble pigment. Due to the presence of carotenoids and anthocyanins, flowers and fruits become brightly coloured. As a result they attract agents to bring about pollination and fruit dispersal, which are the vital phenomenon in plant life.
Enzymes are regarded as ‘biological catalyst’ that catalyzes all the chemical reactions that occur in a plant cell. In a metabolic cell there are many metabolic pathways of different metabolisms like carbohydrate metabolism, protein metabolism, fat metabolism etc. Enzymes catalyze every step of chemical reactions of these pathways. Example: zymase, lipase, protease, invertase, diastase etc.
Nectar is produced and secreted by the nectar secreting glands called nectaries that are present in flowers. Nectars are mainly composed of glucose, fructose and sucrose. Insects are attracted to flowers for obtaining nectar. Insects during their visit carry pollen grains from one flower to another and thus pollination is effected in entomogenous flowers.
3. Excretory material:
These ergastic substances are the by-products of plant metabolisms and are of no use to the plants. These waste materials remain stored in the dead cells. Plants do not possess any excretory system. They get rid of some of the excretory or waste materials through fall of leaves, fruits, seeds, barks etc.
Some of the excretory materials are very much valuable to man. A few important excretory materials are mentioned below:
These are nitrogenous waste substances and are found in roots, leaves, barks, seeds and other parts. Example: quinine occurs in the bark of Cinchona, morphine is present in the fruit of opium poppy (Palaver somniferum), atropine in the leaves of Atropa belladonna etc.
These are the depositions of waste products within a cell. They are composed of either calcium carbonate or calcium oxalate, which is more common. The crystal-containing cell may be similar to neighbouring cell that lack crystals or it may be easily distinguished from the other cells by their shape and size.
This special crystal-containing cell is termed as idioblast. Crystals originate within vacuoles. Crystals may remain single or as aggregate in the cell. Sometimes a large crystal is present filling up the entire cell cavity. Usually they lie loose in the cells, but some are found to be suspended from the cell wall.
i. Crystals of calcium oxalate:
They appear in definite forms that are as follows (Fig. 1.10).
It is rectangular or pyramidal in shape and occurs single or as twin prisms. Example: leaves of Citrus, Vicia sativa, Begonia etc.
It is more or less spheroidal in shape. Many prism or pyramidal crystals aggregate to form a druse. In these crystal-aggregates some projecting points are observed all over the surface. Druse is also known as conglomerate crystal or sphere-crystal or sphaeraphide. They are found in the leaves of Datura, Nerium, petiole of Carica etc.
It is thin elongated needle shaped or acicular crystal that is tapered at both ends to a tip point. It usually occurs as aggregate bundles in idioblast, which is much larger than the neighbouring cells. Raphides are found in the petals of Impatiens, leaves of Colocasia etc. The raphide containing cells may contain mucilage. A matrix of polysaccharide may surround raphides as is found in some members of Araceae.
Electron microscopic study reveals the presence of excrescences in the form of minute bristles in the raphides of Colocasia, whose edible species when consumed causes irritation of mouth and throat. It is assumed that the irritation is due to scratches caused by bristles and pointed ends of raphides.
Pseudoraphides or styloids:
These are elongated prismatic crystals that are tapered into blades at both ends. A cell may contain solitary or a few styloids. They are found in some members of Liliaceae, Agavaceae etc.
In Umbelliferae, the aleurone grains in seeds contain rosette crystals. The crystals are large with equal lengths. They deposit around a centre along the radius in all directions to from rosette.
Crystals of calcium carbonate:
These crystals are known as cystolith and are found in parenchyma, epidermal cells, trichomes or hairs (e.g. Humulus lupulus) and in the leaves of Acanthaceae, Urticaceae, Cucurbitaceae etc. A cell may contain solitary or more than one crystal. The leaf of Momordica contains double cystolith. The epidermal cell containing cystolith is known as lithocyst.
In the leaf of Ficus elastica, the lithocyst that contains the solitary cystolith is much larger than other epidermal cells. In the lithocyst a stalk made up of cellulose develops and projects into the cell lumen. Calcium carbonate deposits around the stalk and fully developed cystolith appears as grape-like clusters with a stalk that is attached to the cell wall from where it hangs into the cell lumen (Fig. 1.11).
During development all epidermal cells have similar appearance. Later, cells destined to become lithocyst can be differentiated by their large nucleus and denser cytoplasm. The cells do not divide along with neighbouring cells, only enlarge and become lithocyst where the nucleus appears to remain functional.
They are very small prismatic crystals and occur frequently in masses in the cell. They are found in Solanaceae and in Atropa belladonna-, each crystal may be spheroidal or wedge shaped. Dormer (1962) pointed out the taxonomic significance of crystals. In an extensive study with 112 species of the genus Centaurea, Dormer was able to segregate the species into two groups on the basis of crystals.
(c) Essential oils or volatile oil:
The odoriferous organs of a plant contain essential oils. They are produced in special cells or glands. They occur in flowers, leaves, and fruits etc. Mention may be made of oil of peppermint from Mentha piperita, clove oil obtained form Eugenia caryophyllata, eucalyptus oil found in Eucalyptus globulus etc. Though volatile oils are excretory products, yet they have the biological significance to attract vectors to bring about pollination and dispersal of seed and fruits.
They are composed of glucose and some other substances, e.g. sinigrin —sulphur-containing glycoside present in Brassica nigra, digitogenin present in Digitalis purpurea etc.
Gums are exudes in the form of thick juice from the bark of Acacia arabica, A. modesta etc. The cellulose of cell wall by decomposition forms gums. Usually gums are exuded naturally, but sometimes they are exuded in response to injuries to heal up the wounds. The gum arabic of commerce is obtained from Acacia Senegal. It is used in medicine, confectionery and sizing and finishing materials in textile industry.
Latex is a fluid secreted by special group of cells termed laticifers. They are present in the families Asclepiadaceae, Apocynaceae, Moraceae, and Euphorbiaceae etc.
Latex may be colourless or coloured as milky white or yellow to orange. It consists of carbohydrate, alkaloids, organic acids, oils, resins, rubber, starch etc. Papain, the proteolytic enzyme is present in the latex of Carica papaya. The alkaloid opium is rich in the latex of Papaver somniferum; Hevea brasiliensis, a member of Euphorbiaceae, yields about 2240 kg rubber per hectare annually.
(g) Organic acid:
Different kinds of organic acids are found in plants and some of them remain concentrated in leaves and fruits. These are produced during different types of metabolisms. Some common acids are tartaric acid — found in tamarind, malic acid present in apple, citric acid in oranges etc.
These may be solid, brittle or liquid in nature with complex chemical composition. They are produced in special gland or secretory cells present surrounding the resin ducts. Most of the coniferous trees are rich in resins. The resin Canada balsam is obtained from the conifer Abies balsamea.
(i) Silica bodies:
These are composed of silicon dioxide and occur in the epidermal cells of Poaceae, Cyperaceae etc.
These are non-nitrogenous phenol derivatives and present in cell sap. They may be yellow, red or brown and appear as granules in a cell. Tannins are found in seed coats, in unripe fruits, in leaves and in the tissues infected by pathogens. Tannins are also present in periderm and heartwood; as a result they become hard. The presence of tannin prevents pathogenic attack. Tannins are used in medicine, in leather industry, in dye industry etc.