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REPRODUCTIVE ORGANS Flower •Flower is the structure concerned with sexual reproduction, consisting of the androecium (male organs: stamens) and gynoecium (female organs: carpels), commonly surrounded by sterile corolla (petals) and calyx (sepals). Flower and reproduction •Flower is composed of 4 groups of modified leaves: sepals, petals, stamens and carpels. •Sepals are the outer floral leaves, usually greenish, which are borne in a tight spiral or whorled. They form calyx which protects flower (forming buds) before opening. Sepals can be separate or fused. Sometimes calyx is persistent and accompanies fruit after fertilization. •Petals are the inner floral leaves, usually brightly coloured or smelling to attract pollinators, and borne in a tight spiral, or whorled. They form corolla which encloses the reproductive organs and, together calyx, protects them. Petals can be separate or fused. After fertilization petals fall down. •Stamens are fertile leaves (sporophylls) forming the male organs comprising a stalk (the filament) and the anthers which are commonly 2-lobed. Anthers contain microsporangia that produce microspores giving rise to male gametophytes. •Carpels are fertile leaves (sporophylls) forming the female organs comprising an ovary (containing 1 to many ovules borne on a placenta) and with a usually terminal style tipped by the stigma. Ovules contain megasporangia that produce megaspores giving rise to female gametophytes (embryo sac). Structure of flowers PERFECT or MONOCLINOUS (BISEXUAL) IMPERFECT or DICLINOUS (UNISEXUAL) Stamens and carpels occur in the same flower Stamens or carpels in separate flowers in the same plant MONOICIOUS PLANT DIOICIOUS PLANT Plant with bisexual flowers, or alternatively with both stamen and carpels in flowers in the same individuals Flowers with stamens and carpels in separate individuals COMPLETE INCOMPLETE With sepals, petals, stamens and carpels in the same flower Lack of one or more floral parts Floral symmetry Regular flowers (Actinomorphic) Irregular flowers (Zygomorphic) Similar elements of whorls, with radial symmetry, i.e. divisible into equal halves in many planes Different element of whorls, with bilateral symmetry, i.e. divisible into equal halves in only 1 plane Morphology of calyx and corolla Morphology of calyx and corolla Origin of the carpel from a reproductive leaf (sporophyll) •A leaf bearing megasporangia folded in order to protect them. Inflorescences Monopodial or indefinite (lateral floral branches arise from a main, central axis) •Raceme or cluster •Spike (spadix, catkin) •Corymb •Umbel •Capitulum Sympodial or definite or cymose (apparent main axis made up of many lateral floral branches, each arising from the one before) •Monochasium •Dichasium Compound (main axis with lateral branches bearing the same or a different type of inflorescence) •Panicle or compound raceme •Compound umbel •Polychasium •Spikelet or compound spike •Cluster of capitula •Cluster of spikes Inflorescences panicle composite umbel spike raceme corymb helicoid monochasium dichasium scorpioid monochasium spadix umbel capitulum polychasium •Monopodial or indefinite inflorescences •Compound and definite inflorescences Monopodial inflorescences •Raceme: the main axis continues to grow, producing flowers laterally with pedicels. •Spike: a raceme in which the flowers are all sessile; is characteristic of Poaceae. •Corymb: a racemose inflorescence in which the lower pedicels are longer than the upper so that the flowers lie as a dome, or dish, and the outline is roundish or flattish. •Umbel: all the pedicels arise at the apex of an axis. It is commonly compound, usually umbrella-shaped, and is characteristic of Apiaceae. •Capitulum: consists of closely packed flowers or florets which have no stalks and arise on a flattened axis, all at the same level. It is surrounded by an involucre of bracts giving it the appearance of a single flower. Capitula are typical of Asteraceae. Cymose and compound inflorescences •Monochasium: consists of a single floral branch bearing flowers and ending in a single terminal flower (e.g. scorpioid and helicoid cymes). •Dichasium: each branch give rise to two more branches. •Polychasium : each branch give rise to many more branches. •Panicle: a compound raceme, i.e. one that comprises several racemose parts; it is characteristic of Poaceae. •Compound umbel: an umbel in which the pedicels give rise to other umbels. •Spikelet or compound spike: a spike in which short branches give rise to 2-3 spikes; it is characteristic of Poaceae. •Cluster of capitula: a racemose inflorescence in which the pedicels give rise to many capitula. •Cluster of spikes: a racemose inflorescence in which the pedicels give rise to many spikes. Inflorescences Inflorescences Inflorescences •Capitulum: consists of closely packed flowers or florets (disc and ray florets) which have no stalks and arise on a flattened axis, all at the same level. It is surrounded by an involucre of bracts giving it the appearance of a single flower. Capitula are typical of Asteraceae. Inflorescences Capitulum of thistle with only tubular, disc florets Capitulum of dandelion with only ligulate, ray florets Capitulum of sunflower with both tubular and ligulate florets Inflorescences •Compound umbel: all the pedicels arise at the apex of an axis; each pedicel gives rise to one umbel. It is characteristic of Apiaceae. Inflorescences simple umbel compound umbel Inflorescences •Spikelet is typical of Poaceae. Inflorescences •Panicles of Zea mays Development of gametophytes sporogenesis gametogenesis Stamens •Stamen is the male organ of a flower, comprising a stalk (the filament) and the anther which is commonly 2-lobed, each lobe containing 2 pollen sacs (microsporangia) in which microsporocytes undergo 2 meiotic divisions to produce 4 microspores that become pollen grains. immature pollen grain Pollen grain •Pollen grain derives from germinating microspore. It contains the male gametophyte, a structure made up of the pollen grain plus a pollen tube. The grain contains 3 haploid nuclei (a tube nucleus and 2 sperm nuclei). Pollen grain • Pollen grain (20-250 μm) is protected by an outer envelope called exin (composed of sporopollenin) showing various types of sculptures (taxonomic value), and by an inner envelope called intin (composed of cellulose and pectins). It bears stretched or circular openings from which the pollen tube comes out. They are known as solcus and porus. • Pollen grains are an important source of nutrients for animals and insects (rich of starch and lipids). • They are frequent in fossils. Various kinds of pollen grains at SEM Carpel •Carpel is the female reproductive organ of the flower, i.e. a unit of the gynoecium, comprising an ovary (containing 1 to many ovules borne on a placenta) and with a usually terminal style tipped by the stigma. •Style is an extension of the carpel which supports the stigma. •Stigma is the part on which pollen grains germinate. c a r p e l Pollination and pollen tube formation •Stigma is the part on which pollen grains germinate through absorption of water and consequent formation of the pollen tube. •Angiosperms have a stigmatic pollination. Pollination and pollen tube formation •The pollen grain contains 3 haploid nuclei (a tube nucleus and 2 sperm nuclei), which pass down the tube to the ovum. One of the sperm nuclei fertilizes the ovum, and the second fuses with the 2 polar nuclei forming the endosperm. The tube nucleus (which is considered to be vestigial, having been completely functional earlier in the evolution of flowering plants) degenerates after double fertilization (so called because of the two unions of nuclei). •With the appearance of flower, angiosperms have solved the problem related to the impossibility to move towards a reproductive partner. •Flower assures a high rate of cross pollination. •Pollination is the transfer of pollen grains (containing male gametophytes) from the anther to the stigma of a flowering plant. Flowers and insects have co-evolved. •Rosemary’s anthers depositing white pollen grains on hairy back of a bee. •Flowers pollinated by bees are yellow or blue since these insects are not sensitive to red wavelenghts. •Flower of Digitalis with tubular corolla showing inside some stains acting as landing platform for bees. •Flowers of the genus Ophrys (Orchidaceae), miming the shape of a female bee, attract male pollinators. •Nectars are an important feeding source for pollinators. They are secretory structures producing energetic nutrients. •Flowers pollinated by birds are often of red colour. •Graminae have an anemophilous pollination: hairy stigmas get pollen grains from hanging anthers. •Wind-pollinated plants do not depend on insects so that they do not spend energy for nectar production, and are colourless and odourless. •Anemophilous pollination is less efficient and succeeds only when a high number of individuals of a population stay close each other (no farther than 100 m). •Some aquatic angiosperms are pollinated by water. •The most important pigments for the colour of flowers are flavonoids (mainly anthocyanins) and carotenoids. •Fertilization involves the fusion of two haploid nuclei from two distinct individuals (cross-fertilization) or from one individual (selffertilization). •Normally male and female organs of a flower mature at different times (dichogamy), or alternatively they are physically separated in order to favour cross-fertilization. •Plants with self-pollination have small, colourless flowers since they do not need to attract pollinators. •Self-pollinated populations have a low level of genetic variability. •Self-pollination is favoured in some cases (e.g. disturbed areas where pollinators are rare). Female gametophyte and double fertilization •The female gametophyte, known as embryo sac, is an oval structure in the nucellus of the ovule of flowering plants, formed by the division of the haploid megaspore nucleus (which is produced by the only diploid cell undergoing meiosis in the megasporangium), and the site of fertilization of the egg and development of the embryo. •It consists of 6 haploid cells without cell walls (2 synergidae, 3 antipodal cells, and an egg cell) and 2 haploid nuclei (polar nuclei). Sometimes the 2 haploid, polar nuclei fuse to form a single, endosperm mother cell. At fertilization, 1 male nucleus fuses with the egg nucleus to form a zygote which develops into the embryo. The second male nucleus fuses with the primary endosperm nucleus to form the endosperm nucleus. This then divides to form the endosperm. Double fertilization •It occurs only in angiospermes and is the production of 2 sperm nuclei, both of which contribute to fertilization. •The male gametophyte, comprising the pollen grain and pollen tube, contains 2 sperm nuclei and 1 vegetative nucleus. •The vegetative nucleus degenerates once the pollen tube has penetrated the embryo sac by micropyle. •The 2 sperm nuclei enter the embryo sac. One unites with the egg nucleus to form the zygote, which develops into the embryo. The other unites with both of the polar nuclei or with the nucleus formed by their fusion, to form the primary endosperm nucleus, from which endosperm develops. •Endosperm is a triploid tissue storing food materials which are broken down during seed germination. •Double fertilization assures endosperm development just in the case of egg cell fertilization, avoiding the waste of nutritive substances. Double fertilization Seed •Seed of angiosperms is composed of 2 diploid generations (the outer integuments and the embryo) and one triploid generation (endosperm). •Embryo has usually 1 or 2 cotyledons. •Endosperm functions as storage of nutrients (carbohydrates, proteins, lipids) for embryo and seed germination. •Seed nutrients are stored in the vacuole or leucoplasts of endosperm cells. Life cycle of an angiosperm Fruit • After fertilization, ovule is transformed into seed, ovary into fruit. • Fruits protect quiescent seeds and contribute to their dispersal. Fruit • Ovary walls known as pericarp, thicken and differentiate into 3 layers: the inner endocarp, the middle mesocarp and the outer exocarp. These layers are usually more evident in fleshy fruits. Classification of fruits origin AGGREGATE SIMPLE MULTIPLE formed by the joining of several carpels that were separate in the flower (e.g. Rubus). formed from a single carpel or from various fused carpels formed from inflorescence pineapple) dehiscence INDEHISCENT remaining closed at DEHISCENT (many-seeded, dry) maturity opening naturally at maturity follicle, legume, lomentum, siliqua, capsule, schizocarp consistence FLESHY (one or more seeds) berry, drupe, pome DRY (one seed) samara, nut, achene, caryopsis an entire (e.g. fig, Classification of fruits TRUE Strictly, the ripened ovary of a plant and its contents FALSE When the ripened ovary and seeds are combined with other structures; e.g. the apple (a pome) in which the true fruit (core) is surrounded by flesh derived from the floral receptacle. Fleshy fruits • Drupe: a fleshy indehiscent fruit containing one or a few seeds, each enclosed within a stony endocarp, as in the genus Prunus (Rosaceae). Fleshy fruits • Berry: a fleshy, indehiscent, fruit containing many seeds immersed in a pulp (endocarp+mesocarp); they do not have hard parts except the seeds (e.g. grape, tomatoes, blueberry, currant, banana, avocado, pepper, eggplant, belladonna). Fleshy fruits • Hesperidium: a berry in which the fleshy part is divided into segments and the outer skin is a tough, glandular leathery rind, e.g. Citrus (Rutaceae such as orange, lemon etc.). Fleshy fruits • Pepper: a berry, typical of Capsicum where the fleshy pericarp surrounds a hollow containing many seeds. Fleshy fruits • Pepo: a unilocular, many-seeded, hard-walled berry that forms the fruit of Cucurbitaceae (e.g. marrow, melon, watermelon, cucumber etc.). Fleshy fruits • Sorosis: a fleshy, aggregate fruit (e.g. mulberry, currant etc.); strawberry is an aggregate, false fruit, where the pulp originates from receptacle while the true fruits are achenes. Fleshy fruits (enlargement of the receptacle) • Pome: fruit in which the seeds are protected by a tough carpel wall and the entire fruit is embedded in a fleshy receptacle; in the apple, the carpel wall surrounding the seeds comprises the core, which is the fruit, the edible fleshy part of the apple being the receptacle. Fleshy fruits • Syncarp: a fleshy, multiple fruit formed from an inflorescence, often including bracts, as in pineapple. Ripening immature • Ripening is a stage in the development of fruit, characterized by the softening, colouring, and sweetening of the tissue, and by a decrease in acidity. Often this is associated with an increase in the rates of respiration, cell expansion, and ethylene production, combined with a loss of chlorophyll. • The colour change is a signal for the seed dispersors. ripening Ripening • Red colour is dominant among ripe fruits since animals, that are sensitive to red wavelenghts, are the best seed dispersors. Dry fruits • Follicle: a dry fruit derived from a single carpel, containing more than one seed, which dehisces along one side (along the suture) only. • Siliqua: the capsular fruit of the Brassicaceae when at least three times as long as broad, containing many seeds attached to a central outgrowth known as replum; it dehisces along dorsal and ventral sides (sutures). Dehiscent fruits • Capsule: a dry, dehyscent fruit formed from a syncarpous ovary; it contains many seeds and opens in different ways. Dehiscent fruits • Legume or pod: a two-valved fruit formed from a single carpel, usually severalseeded and usually opening along dorsal and ventral sides, typical of Fabaceae. Dehiscent fruits • Lomentum: a fruit derived from a single carpel which breaks up into one-seeded portions, as in carob tree and tamarind. Dehiscent fruits • Schizocarp: a fruit derived from a syncarpous ovary which breaks up at maturity into one-seeded portions (mericarps), as in most members of Lamiaceae (4 mericarps) and Apiaceae (2 mericarps). Indehiscent fruits •Samara: a dry, indehiscent, winged 1- or 2-seeded (double samara) nut or achene, characteristic of the genus Fraxinus and Acer. •Achene: a small, dry, oneseeded, indehiscent fruit, strictly of one carpel. Indehiscent fruits • Cypsela: a small, dry, one-seeded, indehiscent fruit formed from two united carpels, normally accompained by pappus, as in the Asteraceae. Sometimes treated as a form of achene. Indehiscent fruits • Nut: a dry, one-seeded, indehiscent fruit with a woody pericarp (e.g. acorn, hazelnut, walnut); the seed is not united to the pericarp. Indehiscent fruits • Caryopsis: a dry, one-seeded, indehiscent fruit, characteristic of grasses (Poaceae), having the pericarp united to the seed; the grain of a cereal grass. Fruit dispersal •Fruits of devil’s claw with hooks joining to fur coat of animals. Pharmaceutical importance of fruits and seeds •The following fruits are used in pharmaceutical preparations: papaya, orange and citrus, anise, star anise, capsicum, senna. •Several plants provide seed containing oils to be used in pharmaceutics: rape, linseed, cotton, castor, coconut, sesame, etc. •Other seed used in pharmaceutics are: coffee, cocoa, sweet and bitter almond, walnut, hazelnut, chestnut etc.