Medicinal Plants PDF

THE CELL AND CELL DIFFERENTIATION The Cell: Cell is the fundamental unite of a living organism (plants and animals). The Cell consists of Cell wall and nucleus appears to be suspended within cell by cytoplasm in which there may be large vacuoles with their own characteristic con...

THE CELL AND CELL DIFFERENTIATION The Cell: Cell is the fundamental unite of a living organism (plants and animals). The Cell consists of Cell wall and nucleus appears to be suspended within cell by cytoplasm in which there may be large vacuoles with their own characteristic contents (crystals and aleurone grains). Other cytoplasmic inclusions are mitochondira, Golgi bodies, lysosomes, and plastids. A group of cells with identical form and function is known as a tissue. The Cell Wall: The wall of mature Cell consists of:  The middle lamella is the intercellular substance, an amorphous, colloidal layer composed of pectic substance.  Primary wall or the original cambial wall is composed of cellulose and pectic substance.  Secondary wall is formed after the enlargement of the cell has been ceased. It consists mainly of cellulose associated with polysaccharides. It undergoes to some modifications as desposition of various chemical substance as lignin, suberin and cutin.  Tertiary wall: It is the innermost layer of the cell wall and is usually thin, highly refractive and formed of cellulose. Properties of different cell walls: A. Cellulosic Walls: Formed mainly of cellulose which may be accompanied with hemicellulose and pectin. Cellulose is polysaccharide, composed of linear chain of glucose residue. Chemical Properties:  Give blue color with iodine and sulphuric acid.  Give blue color with chloro- Zinc- iodine.  Give no color with aniline or phloroglucin and HCL.  They dissolve in ammonical solution of copper oxide (Cuoxam) and precipitated with dilute sulphuric acid. 2/ 98 B. Mucilaginous Walls: Certain cellulosic cell walls may be converted into gums and mucilages. Mucilage is polysaccharides, consisting of sugar and uronic acid combined with metals. Chemical Properties:  They are variably stained with ruthenium red, iodine sulphuric acid or corallin soda. C. Lignified Walls: Lignin is a strengthening material. Chemically it is a complex phenylpropaniod polymer. Chemical Properties:  Stain magenta red with phloroglucin and hydrochloric acid. D. Suberised and cutinized walls: Suberin and cutin are mixture of polymerized fatty acids and suberic acid Chemical Properties:  Stain red with alkanna tincture and sudan III E. Chitinous walls: Chitin is a polysaccharide derivative containing acetyl and amino group. It constitutes the organic skeletal substance of insects and many fungi. When heated with 50 % potash at 160 – 170 o C for 1 hour, it is converted into chitosan C14H26O16N2, ammonia and acids such as acetic and oxalic. Chitosan gives a deep violet coloration when treated with iodine solution followed by dilute sulphuric acid. 3/ 98  Cell Differentiation Plant tissue may be:  Paranchymatous Tissues: Is the fundamental or ground tissues of the plant. They are cellulosic, isodiametric orslightly axially elongated cells with narrow or wide intercellular spaces. The cells show different contents e.g. starch granules, calcium oxalate crystals. They constitute most of the pith and cortex of the plant.  Collenchymatous Tissues: It is a living mechanical tissue, composed of rounded or elongated cells, with unevenly thickened, nonlignified walls. The thickening is of cellulose and located commonly in the angles (angular collenchyma) or chiefly on the tangential walls (lamellar collenchyma). Collenchyma constitutes the typical mechanical tissue of the herbaceous stems, and of the petioles and midribs of leaves. 5/ 98 6/ 98  Sclerenchymatous Tissues: They are thick walled; dead, lignified celled mainly for mechanical functions, consist of sclereids and fibres. o Sclereids: They are thick, lignified cells with narrow to wide lumen and isodiametrical in shape or elongated but the length is never many times as the breadth. o Fibres: They are dead, very much elongated, thick, lignified, pitted, polygonal cells usually with tapering pointed ends. They may have a tortuous irregular outline. 7/ 98 Secretory Tissues: They may be: External: e.g. glandular hair. Internal: include secretory cells, secretory cavities or glands secretory ducts or canals and latex tissues. Secretory Cells: They occur either isolated or rarely in rows having suberised walls. They differ from the other cells; by their contents and size. The cells are named after the secretion they contain. The secretion may be volatile oil (oil cell), resin (resin cell), gum resin, mucilage, enzyme or tannin. Secretory Cavities (Glands): These are internal structures, embedded in a mass of tissue and having cavities within them in which secretions e.g. volatile oil and oleoresin, etc are secreted. According to method of formation they may be:  Schizogenous: Developed by splitting a part of cells thus enclosing a cavity which then become enlarged and lined by epithelial cells formed by the division of the surrounding cells e.g. savin.  Schizolysigenous: Developed at first schizogenously but, later on, increase in size by breaking down the bounding cells e.g. clove, Buchu.  Lysigenous: Developed by breaking down of the cells forming a cavity. It originates from single cell by the division in different direction forming a solid mass and then by the gradual disintigration of the resulting cells starting from the center e.g. Dictamnus. Secretory Ducts: They are tube like structures which may extend through the whole length of the organ e.g. leaf pinus, or even through the whole of the plant. They are developed either schizogenously as vitta of Umbelliferse, lysigenously or schizolysigenously as in Capaifera. Laticiferous Structures: These include tube-like cells and vessels with a colorless, milky white, yellowish or reddish viscous emulsion called latex. 9/ 98 Latex may contain resin, gum-resin, fat wax, proteins etc. suspended in an aqueous solution of alkaloidal salts, tannins minerals, sugar enzyme etc.  Latex Cell: Usually in the form of tube, branched as in Euphorbia or unbranched as in Cannabis but not anastomosing e.g. Euphorbiaceae and Apocynaceae.  Latex Vessel: They are long simple or branching tube formed by the partial or complete fusion of the transverse walls of a longitudinal series of cells. They are anastomosing with rough walls e.g. Papaveracea and Compositae. 10/ 98   Primary metabolic products. (Food storage products) e.g. carbohydrates, proteins and lipids.  Secondary metabolic products (by-products of metabolism) e.g. alkaloids, glycosides, volatile oils, resins and tannins.  Crystalline mineral products e.g. calcium oxalate and calcium carbonate. Primary metabolites: I. Carbohydrates: are organic compounds composed of carbon, hydrogen and oxygen, where the last two elements are present of the ratio 2: 1 like that of their presence in water. They include starch and sugar.  Starch: Are polysaccharides of higher molecular weight, with the general formula (C6H10O5)n. It is formed as a result of the photosynthesis process which carried out by the green parts of the plant. The plant takes carbon dioxide from the air and absorbs water from soil and in the presence of chloroplasts and sunlight energy it forms glucose. sunlight 6 CO2 + 6 H2O C6H12O6 + 6 O2 The formed glucose molecules are condensed together with the elimination of water to give starch. It occurs in granules of varying size in almost organs of the plants, either alone or accompanied with other reserve food particles, e.g. proteins. Chemical Composition of Starch: Starch granules consist of:  Amylose: It forms the inner layers of the granule and is soluble in water.  Amylopectin: It forms the outer layers of the granule and is insoluble in water. 12/ 98 Macroscopical Characters:  Starch occurs in irregular, angular masses or as a white powder.  It is insoluble in cold water but forms a colloidal solution on boiling water, the solution forms translucent jelly on cooling.  Starch granules also undergo gelatinization when treated with caustic potash, hydrochloric acid, concentrated solutions of calcium or zinc chlorides or concentrated solution of chloral hydrate.  Starch is colored deep blue with solution of iodine; the color disappears on heating but reappears on cooling.  When starches are heated with water, the granules first swell and then undergo gelatinization. Microscopical Characters: Starches can be identified by microscopical examination when mounted in water or lactophenol. The Shape: vary from polygonal, round to oval. The hilum: It is starting point of formation of the granule in the leucoplast, it may be centric or eccentric, the hilum takes the form of a round point simple, curved or multiple cleft. The Striation: are alternate layers around the hilum, either concentric rings or transverse. The aggregation: may be simple or compound. Some of the more important microscopic characters of the principal starches are set out in the following table. 13/ 98  CELL CONTENTS The cell contents are those substances inside the plant cell and can be identified by microscopical examination or by chemical and physical tests. They include: 11/ 98 Comparison between the four important commercial starches in their microscopical features: Item Potato Wheat Maize Rice polyhedral Oval, rounded, Oval, rounded, with Polyhedral with Shape lenticular from side pyramidal rounded sharp angles view angles Size Large Medium Medium small Absent, except in Present, Present, Present, centric, large, simple Hilum eccentric, centric, cleft pointed granules, centric pointed or fissured pointed Distinct, Striation faint, concentric Absent Absent concentric Simple, Mostly Mostly compound, Mostly simple, few, Aggregation simple, few compound, few semi compound compound simple compound Chemical Tests:  Starch gives blue color with iodine solution.  Molische’s Test: suspension of starch and alcoholic α-naphthol in a test tube gives violet ring at the interface upon addition of concentrated H2SO4 on the wall of the test tube.  Starch gives a positive reduction test e.g. Fehling test, after hydrolysis with acid and neutralization of the solution. Uses:  Externally as dusting powder to allay itching or demulcent.  Internally as: o Suspending agent for barium meals before x-ray. o Antidote in the treatment of iodine poisoing. 14/ 98  Pharmaceutical as a diluent, lubricant and a tablet disintegrant.  Commercially: o It is used for paper and cloth sizing. o Nutrient. o Preparation of glucose, dextrose and dextrins. 15/ 98 II - Protiens: Proteins are nitrogenous organic compounds formed from polymers of amino acid units. In plants, protein is stored in the form of amorphous mass called aleurone grains. Aleurone Grains: They are small solid protein particles, commonly present in the seeds. The typical aleurone grains consist of:  Ground Substance: is an amorphous protein, usually soluble in water.  Crystalloid is formed of albumins, usually hardly soluble in water. It is stained brown with iodine solution, yellow with picric acid and red with Millons reagent.  Globoid consists of globulin combined with calcium and magnesium salts of hexaphosporic acid. It is insoluble in water. Examination of aleurone Grain:  The material, either in section or in powder form, is defatted first by maceration in alcohol-ether mixture or then stained. The defatted material is placed in picric acid solution for few minutes, washed with alcohol to remove the excess of stain. Then add alcoholic solution of eosin and finally wash well with alcohol. The stained aleurone grain show yellow crystalloid, colorless globoid and red ground substance.  Proteins give red color with Millons reagent. III - Lipids: They include fixed oils, fats and waxes. These substances are widely distributed in the plant, commonly in the seeds and fruits. Fixed oils are esters of unsaturated, higher molecular weight fatty acids with glycerols. (Liquids) Fats are esters of saturated, higher molecular weight fatty acids with glycerols. (Solids) Waxes are esters derived from higher monohydric alcohol combined with fatty acids. Fixed oils and fats are hydrolyzed by aqueous caustic alkalis to give soap and glycerol. (Specification). 16/ 98 Microscopical Identification of Oils:  The oil globules are colored black with 1 % solution of osmic acid.  The oil globules are colored red with sudan III. Secondary Metabolites I - Alkaloids: Alkaloids are naturally occurring organic compounds containing one or more nitrogen atoms in a heterocyclic ring, usually basic and have marked physiological action on man or animals. The term alkaloid means originally alkali-like. Alkaloids are present in plants either as free bases or as salts of some acids. Characters:  Free bases are sparingly soluble in water but soluble in organic solvents while the alkaloidal salts are soluble in water and sparingly soluble in organic solvents.  Most alkaloids are precipitated from neutral or slightly acidic solution by Mayers reagent giving yellowish precipitate, by Wagners reagent giving reddish brown precipitate, by Dragendorffs reagent giving reddish brown precipitate. Caffeine and other purine bases do not give these precipitates. II - Glycosides: Naturally occurring organic compound of plant organ consisting of sugar part (glycone) and non sugar part (aglycone). Classification: There are four different systems for classification of glycosides:  According to the linkage between glycone and aglycone e.g. O- glycosides, S-glycosides and C-glycosides.  According to pharmacologic action e.g. cardiac glycosides, laxative glycosides etc.  According to sugar moiety e.g. glucoside, rhamnoside, glucorhamnoside etc.  According to aglycones e.g. phenolic, anthracene, flavone etc. 17/ 98 Properties of glycosides:  Glycosides are polar compounds soluble in polar solvents.  Glycosides can be hydrolyzed by acids or enzymes. III - Volatile Oils: Volatile or essential oils are volatile in steam. They are secreted in oil cells, in secretion canals or cavities or in glandular hairs. They are generally mixtures of hydrocarbons and oxygenated compounds derived from these hydrocarbons. Volatile oils are used for flavoring, in perfumery, as spices, and for their therapeutic action as antiseptic, antispasmodic and carminative. Volatile oils are not saponified by alkalies and are detected by sudan III. IV - Tannins: They are water soluble polyphenol compounds present in plant extracts which are able to combine with proteins of animal hides to prevent their putrefaction and convert them into leather. Most true tannins have high molecular weights. Uses of drugs containing tannins:  Astringents in the gastrointestinal tract and on skin abrasions (hemostatic).  In the treatment of burns, where proteins of the exposed tissues are precipitated and form a mildly antiseptic, protective coat under which regeneration of new tissues may take place.  Tanning of leather.  Antidotal treatment of alkaloidal and heavy metals poisoning.  Preparation of ink. Tannins are classified into two main groups: o Hydrolysable Tannins: These may be hydrolyzed by acids or enzymes such as tannase. They are formed from several molecules of phenolic acids such as gallic. On dry distillation, gallic acid and similar components are converted into pyrogallol 18/ 98  so they termed pyrogallol tannins. The hydrolysable tannins give blue black color with ferric chloride. o Condensed Tannins: these are not hydrolyzed to simpler molecules and they do not contain a sugar moiety. Condensed tannins when treated with acids or enzymes are converted into red insoluble compounds known as phlobaphenes. Condensed tannins on dry distillation they yield catechol and these tannins are therefore called catechol tannins. They give green color with Ferric chloride. Pseudotannins: Are compounds of lower molecular weight than true tannins, they also do not respond to the gold to the gold beater s skin test. Test of Tannins:  Gold beater’s Skin test: o Soak a piece of goldbeater skin (membrane of ox-intestine) in 2% HCL then rinse with water. o Place the soaked piece in the solution to be tested for 5 minutes. o Wash with distilled water and transfer to 1 % solution of ferrous sulph1ate. A brown or black color on the skin denotes the presence of tannins.  Ferric Chloride Test: o Tannins are extracted from vegetable powdered plant with hot water; add few drops of ferric chloride solution a blue color is produced with hydrolysable tannins and green color with condensed tannins. Calcium Oxalate It is a common plant cell content, formed as an end product of metabolism. It is found in more or less crystalline form. They are usually useful for the microscopical examination of the drugs as well as detection of adulteration. Characters:  Calcium oxalate crystals are insoluble in water. 19/ 98  TAXONOMY OF MEDICINAL PLANTS Taxonomy: Is a science includes identification, nomenclature and classification of object, usually of biological origin when limited to plants; it is referred to as systematic botany. Botany: Is the scientific study of plant life. It is called plant science, phytology or plant biology. Medicinal Plants: They are those plants used to cure or prevent a disease. They exert their action through certain chemical substances called active constituents. The medicinal herb is collected from annual, biennial and perennial plants. Annual: that lives for one season or one year and dies after production of flowers, fruits and seeds. Biennial: that lives for two seasons but not for more than two years Perennial: that lives for more than two years or indefinitely. Naming of Plants: According to the accepted rule suggested by Linneaus, the binomial system is used in which each plant is given two names, the first indicates the genus (generic name) and always starts with a capital letter, and the second represents the species and always starts with a small letter. Taxonomical Classification of PlantKingdom: The classification of this system depends upon the following:  Individual plants similar in structure and life history are grouped into species.  Different species possessing general similarities to one another are grouped into genus.  Similar genera are grouped into a family. 56/ 98   Closely related families are grouped into an order.  Related orders are grouped into a class.  Related classes are grouped into a division. The plant kingdom is divided into five phyla: Thallophyta, Bryophyta, Pteridophyta, Gymnospermae and Angiospermae. These five phyla are usually included under two big divisions; the Cryptogams (flowerless) or plants with hidden sexual organs which include the first three phyla, and Spermatophyta (or Phanerogams) including the last two phyla which PlantKingdom Cryptogams Phanerogams allophyta Bryophyta Pteridophyta Gymnospermae Angiospermae Algae Fungi Lichens Bacteria Monocotyledone Dicotyledones comprise the flowering plants. Phylum Thallophyta The plant body is undifferentiated into stem, root and leaves and it is called the thallus. They reproduce both sexually and asexually. Most members are aquatic plants. It includes the sub-phylum algae, fungi, lichens and bacteria. Phylum Bryophyta Plants of this phylum show the beginning of alteration of generation both sporophyte and gametophyte are combined in one plant. The sporophyte is parasitic on gametophyte. 57/ 98  Phylum Pteridophyta Plants of this phylum show distinct alteration of generation. Sporophyte stage is larger, segmented into stem, leaves and roots and possesses true vascular bundles. Phylum Gymnospermae Plants of this phylum bear naked ovules (ovules are not enclosed in ovaries but lie on the surface of the sporophyll). Embryos have two to many cotyledons. Phylum Angiospermae It includes the flowering plants. Ovules are enclosed in ovaries. According to number of cotyledons, it is divided into Monocotyledoneae and Dicotyledoneae. Phylum Thallophyta  This phylum includes most primitive plants  Plants are small and many of them live in water (aquatic).  The plant (thallus) is not differentiated into stem, root and leaves.  Reproduction may be sexual or asexual (by spores). Thallophyta includes four subphyla; algae, fungi, lichens and bacteria. Subphylum Algae  Algae are thallus plant (not differentiated into stem, leaves and root) contain chlorophyll and other pigments, live on land or in water.  They are classified into 3 orders according to their color  Chrysophytae (green algae) e.g. Diatoms (which is used for clarification, filtration and decolourization of fluids).  Pheophytae (brown algae) e.g. Fucus and Laminaria. (which is used for preparation of alginates which are used as tablet distintigrants).  Rhodephytae (red algae) e.g. Carrageen and Agar Agar (carrageen has demulcent properties, used in pulmonary 58/ 98  complaints and chronic diarrhea. Agar used in bacteriologic cultures and as laxative. Subphylum Fungi Fungi are non-vascular plants containing no chlorophyll; hence they cannot build their own food. They are either parasitic (on living organisms), saprophytic on dead organisms (living on decaying animal and plant matters). Some fungi are unicellular (as yeast) or have their bodies formed of thread like filaments, known as mycelia. Cell wall is usually built up of a nitrogenous material known as chitin. Reproduction takes place either sexually or asexually. Economic importance of Fungi:  Some Fungi are used in alcoholic fermentation, alcoholic beverage industry (beer)  Some are involved in organic acid production (gluconic, citric, lactic etc).  Certain fungi are used in production of antibiotics or vitamins.  Some are used in synthesis of cortisones and steroidal hormones. Fungi are classified into the following orders according to the mode of reproduction:  Phycomycetes (Algal Fungi) (saprophyte)  Hyphomycetes (Imperfect Fungi) (devoid of a known sexual stage)  Ascomycetes (Sac fungi) e.g. penicillium and Ergot. Penicillium produces penicillin which is antibiotic used for treatment of some diseases. Ergot contains alkaloid Ergometrine which has oxytocic action and the alkaloid Ergotamine is used as an analgesic in migraine.  Basidiomycetes (Club fungi) e.g. Mushrooms. 59/ 98  Subphylum Lichen These are fungi and algae living together in symbiosis with each other. PHYLUM GYMNOSPERMS They are group of woody plants. They differ from Angiosperms in several aspects:  They bear naked ovules on the edges or flat surfaces of leaves called carpels, while Angiosperms bear covered ones.  Each megaspore or embryo sac produces within itself a bulky prothallus, in the upper portion of which originates one or more archegonia, while in Angiosperms no recognizable prothallus exists.  The stored food tissue within their seeds is prothallial tissue loaded with starch, etc., while in Angiosperms seeds, endosperm is developed from the endosperm nucleus.  The mode of growth of their stems is always indefinite, while that of Angiosperms is either definite or indefinite. Pollination is by wind; pollen grains are born to micropyle of naked ovule. PHYLUM ANGIOSPERMS Angiosperms are characterized by:  The ovule are not exposed directly to the air but enclosed in the ovary of the carpel.  Pollen grains are received on the stigma and pass through the style to the ovary to fertilize the ovule (not directly on ovule as in Gymnosperms).  The flowers are arranged on a shortened axis to bring the floral leaves close together and are not separated at different levels along the axis as in Gymnosperm.  The stamen and carpels are absolutely essential, the calyx being largely protective and usually green, the corolla usually serving to attract insects that help in cross pollination.  Angiosperms are mostly herbaceous, annual or biennial forms, whereas Gymnosperms are mostly woody perennials. 60/ 98  The embryo may be straight or variously curved, large or small, and more or less embedded in the endosperm or occupying the whole seed.  The vascular bundles of the stem are collateral or bicollateral, open or closed. According to the number of cotyledons, this phylum is subdivided into subphyla; Monocotyledons, with a single cotyledon and dicotyledons with two cotyledons. Item Dicotyledones Monocotyledones Habit Herbaceous or woody. Mostly herbaceous. Leaves shape commonly Leaves shape characteristic oblong or linear, vennation, for families or species, Leaves parallel, often sheathing at venation,reticulate, petiolate the base, sessile, entire , stipulate leaf sheath absent. margin and extipulate. Roots Tap Root adventitious Usually consists of a ring of Closed, collateral or open primary bundles with a concentric of numerous Vascular cambium, stem scattered bundles, no System differentiated into cortex and differentiation into cortical stele. and stellar regions in stems. Parts are usually in tetra or Parts are usually in Trimerous Flower penta merous. or multiples of three. 61/ 98  Basically tricolpate (having Basically monocolpate Pollen three furrows or pores). (having one furrow or pore). Embryo Two cotyledons. One cotyledon. Is wanting (if present, they Secondary Due to activation of form new closed v.b not due growth cambium. to cambium). The major differences between these two classes are summarized in the following table: 62/ 98  SUBPHYLUM MONOCOTYLEDON Family Zingiberaceae They are perennial herbs usually with fleshy rhizome and thickened roots. The leaves are large elliptical with parallel venation, and may be radical with a sheath that is folded, giving the appearance of a stem. The flowers are hermaphrodite, Zygomorphic, trimerous. The perianth is differentiated into two trimerous whorls that form caylex and corolla. Ovary inferior, trilocular with axile placenta or unilocular with pariatal placenta. The fruits are trilocular capsule. Seeds are albuminous with perisperm and endosperm, rich in starch, show a raphe and arillus. Embryo is small and embedded in the endosperm. The seed shows sclerenchymatous layer in the testa. The presence of aromatic and pungent principles and secretion cells containing volatile oil is characteristic feature of the family. Examples of medicinal plants belonging to this family and their use are:  Cardamom: Used as aromatic, carminative and flavoring agent.  Ginger: Used as condiment, carminative, aromatic stimulant, respiratory and gastro intestinal disorders. SUPHYLUM DICOTYLEDONS Family SOLANACEAE or Night Family  Morphological Characters: They are annual herbs or shrubs with cymose inflorescence.  The leaves are alternate or opposite, exstipulate and often hairy, entire or more or less lobed and adnated to stem.  The flowers are grouped in cymes. They are actinomorphic, pentamerous and perfect. The stamen is epipetalous. The ovary is bicarpellary, syncarpous. Superior, the style is simple and the stigma is simple and bilobed. 64/ 98   The fruit is capsule or berry.  The seeds are albuminous with curved or coiled embryo.  Anatomical Characters:  The stomata are anisocytic.  The non-glandular trichomes and the characteristic club-shaped glandular ones (clavate).  Presence of perimedullary phloem.  Absence of special internal secretory tissue.  Presence of calcium oxalate in different forms.  Plants are rich in mydriatic alkaloids. Plants belonging to this family are:  Hyoscyamus: used as an antispasmodic and sedative.  Capsicum: used as antirheumatic.  Tobacco: used as insecticide. 65/ 98 Structure of the Stem: The young or herbaceous stem of Dicotyledons usually shows the following layers from outside inwards:  Epidermis: is the outermost layer.  Cortex: limited by an endodermis.  The Stele: Includes all the tissues within the endodermis the pericycle, the vascular tissues, the pith and medullary rays.  The pericyclic: formed of parenchyma or fibers and or sclereids. Vascular Tissue: Consists of a number of vascular bundles arranged in circle around the pith and interrupted by medullary rays. The vascular bundles may be:  Collateral: Formed of an outer phloem and inner xylem. It may be open (with cambium) or closed (devoid of cambium)  Bicollateral: With phloem above and below, the cambium will be present in between the xylem and the outer phloem.  Concentric: with no cambium. Phloem: Consists of sieve tubes and companion cells with or without phloem parenchyma. Cambium: Meristematic tissue.  It is present in open vascular bundles only. It may be restricted in between the phloem and xylem where it is called intrafascicular or may be crossing the medullary rays i.e. interfascicular Xylem: It is generally lignified. It is always endarch i.e. having the protoxylem directed inwards and the metaxylem outwards. Xylem consists of vessels of various types of thickening wood fibres, wood parenchyma and tracheids. Vessels are few and arranged to form Y- shape in case of Monocotyledons. 24/ 98 Pith: It consists of parenchyma in the young stem but usually lignified and pitted in the old stem. In monocotyledons, a large number of closed vascular bundles scattered irregularly through the ground tissue, which is not marked off into pith and cortex. Secondary Thickening: Means increase in diameter. This increase is mainly due to formation of new vascular tissues by the successive division of the cambial cells giving to the outside secondary phloem and to the inside secondary xylem. But, in 26/ 98 the Monocotyledons, the enlargement is mainly due to the expansion of primary elements as there is no cambium or by the increase in the number of vascular bundles. In the secondary thickening of Dicotyledones, some cells of the primary medullary rays are activated to form interfascicular cambium connecting the intrafascicular cambium to form a continuous ring which starts to give secondary phloem outwards and secondary xylem inwards forming a continuous ring of vascular tissues. Occasionally, the interfascicular cambium divides to give only parenchyma and thus the bundles remain separated by wide primary medullary rays. The development of the secondary vascular elements increase in the diameter of the stem which subjects the outer protective layers to rupturing and is, therefore, usually accompanied by the formation of cork cambium, phellogen, and the consequent formation of periderm. 27/ 98  SUBTERRANEAN ORGANS The subterranean organs (the underground organs) of the plant are either of the stem origin as rhizomes, corms, bulbs and tubers or of root origin such as roots and root tubers. A- The Root: The root is the portion of the plant axis grows down wards into the soil, characterized by:  It never develops leaves.  It shows no nodes and internodes.  It bears no buds.  It shows no chlorophyll. Function of Root:  Absorption of water and minerals from the soil to the stem.  Support the plant to the ground.  Serve as storage organ. Types of Roots: 1. Primary Roots: Developing into tap-root. 2. Secondary Roots: Lateral branches of tap –root. 28/ 98 3. Adventitious Roots: Arise from the stem. 4. Fibrous Roots: Very slender. 5. Storage Roots: Roots swollened with reserve food material. 29/ 98 Structure of Root:  Structure of young Root: o Piliferous Layer: It is formed of a single row of thin walled cells showing no stomata, no cuticle and no intercellular spaces. The cells bear and produce root hairs. o Cortex: Parenchymatous of an outer layer called exodermis. o Endodermis: is the innermost layer of the cortex. o Stele: Consists of pericycle enclosing the vascular bundles. The vascular bundles are radial formed of alternating groups’ phloem and xylem separated with conducting parenchyma. It shows no cambium or central pith. The xylem occurs in radial groups or arches with the protoxylem directed outwards that is called exarches.  Structure of old root: The cambium is originated in the parenchyma below the phloem, in the conducting parenchyma and in the pericycle outside the protoxylem arches forming a stellate ring. The cambium divides to give secondary phloem outwards and much more xylary tissue, secondary xylem, inwards. At the same time, phellogen is formed in the pericycle dividing to give outer cork and inner wide phelloderm (periderm). As a result of secondary thickening, the old dicotyledonous root is formed of the following tissues: o Cork. o Phelloderm. o Collapsed primary phloem. o Secondary phloem. o Cambium. o Secondary xylem. o Primary xylem groups in the center. 30/ 98 B- Subterranean Stems: The subterranean stem differs from the aerial stem in the following aspects:  It bears scale and not foliage leaves.  It bears adventitious roots arising generally from the nodes.  It has growing point protected by scale leaves. Moreover, it differs from the root as follows:  It generally, has nodes and internodes.  It bears scale leaves with axillary buds.  Numerous adventitious roots are generally arising from the nodes.  It has growing point protected by scale leaves.  Histologically, it shows a central pith Function of Subterranean Stem:  They function as a mean of perennation.  They serve as storage organs for reserve food materials. Modes of branching:  Monopodial: the main axis continues growing and produces the successive yearly portions of the axis.  Sympodial: the main axis stops growing as a result of destruction of the apical bud. The main axis continues its growth by the development of the axillary bud in the axial of scale leaves. Types of Subterranean Stems:  Rhizome: It shows nodes and short or long internodes, terminal bud and aerial shoot, usually thick and branched.  Stem-tuber: swollen with reserve food material, shows small scales and buds.  Corm: It is a shortened swollen erect under ground base of the stem, covered with scale leaves and usually have a large apical bud and 32/ 98  small axillary ones. It usually produces a daughter stem above the parent corm.  Bulb: Reduced flattened discoid under ground stem crowned with crowded fleshly scale leaves or their bases and having adventitious roots. Structure of Subterranean Stems: Subterranean stems generally resemble in structure the aerial stems:  The tegumentary Layer: is the outer protective layer and may be: o Epidermis. o Cork is few rows of regular layers of suberised or lignified cells with no intercellular spaces. o Metaderm consists of few rows of irregularly arranged suberised cells derived from the outer layers of the cortex so may show intercellular spaces.  The Cortex: is parenchymatousfilled with reserve materials.  The Endodermis: It is usually indistinguishable in Dicotyledones.  The vascular bundles: o Open: in Dicotyledones. o Closed in Monocotyledones and the endodermis is distinct. 33/ 98