Presentation on theme: "The Flower Sterile and fertile reproductive organs borne on an axis (the receptacle). A modified shoot exhibiting determinate growth (the floral meristem."— Presentation transcript:
The Flower Sterile and fertile reproductive organs borne on an axis (the receptacle). A modified shoot exhibiting determinate growth (the floral meristem ceases activity after all the floral parts have been produced). The parts are arranged in a whorl rather than a spiral or helix. Cohesion of members of a whorl – the whorl grows as a unit. Adnation of one whorl to another – two or more whorls grow as a unit. Sterile parts – sepals forming the calyx and petals forming the corolla. Calyx and corona make up the perianth. If the sepals and petals are not distinctly different then the members are called tepals. Reproductive parts – stamens (microsporophylls) and carpels (megasporophylls). The stamens constitute the androecium and the carpels constitute the gynoecium. Zygomorphy: zygomorphic flowers have bilateral symmetry. Actinomorphy: actinomorphic flowers have radial symmetry. Inferior ovary (epigyny, epigynous ovary)) – sepals, petals and stamens above the ovary - a more advanced condition than the superior ovary (hypogyny, hypogynous ovary) in which the sepals, petals and stamens occur below the ovary. Perigyny: an extension above the receptacle resembling a cup (receptacular or appendicular (floral tube)) bears the sepals, petals and stamens. Imperfect flower: unisexual, lacking either the gynoecium (staminate flowers) or the androecium (carpellate or pistillate flowers). Flowers are grouped into inflorescences or occur singly at the axis terminus. Sepal and Petal Sepals and petals resemble leaves in structure – they consist of parenchyma, have a more or less branched vascular system and an epidermis. Crystal-containing cells, laticifers, tannin cells and other idioblasts may be present. Young petals may contain starch. Green sepals contain chloroplasts but rarely have differentiated palisade and spongy mesophylls. Petals contain pigments in chromoplasts (carotenoids) and in the cell sap (flavonoids – anthocyanins). Some of these pigments may radiate in the UV. Epidermal cells of petals often contain volatile fragrant oils. The epidermis of both sepals and petals may have stomata and trichomes.
Idealised Flower Asteraceae (Daisies and sunflowers) – a flowerlike inflorescence of florets.
Floral diagrams. A) symbols; b) adnation, connation; c) Lamium album, half flower; d) petals alternate with sepals; e) floral diagram, Lamium album; f) floral diagram of terminal cyathium of Euphorbia sp. A: axis. Br: bract. Bra: bracteole. Ff: female flower. Mf: male flower (stamen only). Ms: missing stamen. O: ovary. Pe: petal. Se: sepal. Si: introrse stamen. Sp: sepal adnate to stamen. Sx: extrorse stamen. Ugs: united glandular stipules of bract. Up: petal connate to petal. Floral Formulae Floral formula: a code indicating the number of flower parts, whorls, the attachment of parts and the nature of the gynoecium. E.g. Lamium album (white dead nettle): K(5) [C(5) A4] G(2) zygomorphic flower, actinomorphic spiral and not whorled parts, K = calyx, C = corolla, A = androecium, G = gynoecium. Numbers show number of members in a whorl, brackets that they are united. Square brackets or bridging lines show two separate whorls whose members are joined. Bar above or below G indicates a superior or inferior ovary respectively. Floral Diagrams
Diversity of Flower Form The calyx is usually green (sepaloid) but maybe coloured (petaloid). It may be regular, zygomorphic or irregular. It may be polysepalous (sepals free) or gamosepalous (sepals united). In the Mussaenda flower one of the sepals becomes a distinct leaf-like structure, often white or coloured. The calyx may be modified into a pappus of spike-like sepals. Caducous calyx: falls off soon after the floral bud opens. Deciduous calyx: the calyx falls when the flower withers. Persistent calyx: remains adherent to the fruit (it may whither, grow into a cup, become coloured, become fleshy or enclose the fruit). The corolla may be regular (radial symmetry), zygomorphic (bilateral symmetry) or irregular (asymmetrical) and may be gamopetalous or polypetalous. Regular and polypetalous corollas 1. Cruciform: 4 free petals, each comprising a claw and limb (blade) and in the form of a cross (Cruciferae, e.g. mustard, radish, cabbage, cauliflower, etc.). 2. Caryophyllaceous: 5 petals with long claws and limbs at right-angles to the claws, e.g. Dianthus. 3. Rosaceous: 5 petals with very short claws or no claws and limbs spread regularly outwards, e.g. rose, tea, prune. Regular and gametopetalous corollas 1. Campanulate: bell-shaped corolla, e.g. gooseberry (Physalis), bell flower (Campanula). 2. Tubular: corolla cylindrical or tube-like, e.g. central sunflower florets. 3. Infundibuliform: funnel-shaped corolla, e.g. morning glory. 4. Rotate: wheel-shaped corolla – narrow and short, limb at right-angles to the tube, e.g. jasmine (Jasminium). 5. Hypocrateriform: salver-shaped corolla – a rotate form with a long corolla-tube, e.g. periwinkle (Vinca). Zygomorophic and polypetalous corollas Papilionaceous: butterfly-like – 5 petals, the outermost petal (standard or vexillum) is the largest; two lateral wings or alae; the two inner ones are the smallest and form a boat- shaped cavity (keel or carina), e.g. pea, bean. Zygomorphic and gamopetalous corollas 1. Bilabiate: two-lipped (upper and lower lips0 with a gaping open mouth, e.g. basil (Ocimum). 2. Personate or masked: two-lipped but with a closed mouth. The palate is the projection of the lower lip that closes the mouth, e.g. snapdragon (Antirrhinum). 3. Ligulate: strap-shaped – the corolla forms a short, narrow tube below and is flattened above, e.g. outer sunflower florets. Cyclic flower: sepals, petals, stamens and carpels arranged in circles or whorls around the receptacle (most flowers) and acyclic when these are arranged in spirals (e.g. water lily and Magnolia). Hemicyclic flower: some parts are cyclic, others acyclic, e.g. rose.
Appendages of the corolla and perianth In snapdragon the corolla tube is slightly dilated on one side like a pouch and is saccate or gibbous. In orchids, the perianth is prolongated into a nectar-containing tube, called a spur (spurred perianth). The corolla may split transversely to form an extra whorl of lobes, scales or hairs, free or united, called the corona (crown), e.g. Passion flower (Passiflora). The daffodil (Narcissus) has a cup-shaped corona. The corona helps attract pollinating insects. Aestivation Aestivation is the mode of arrangement of the sepals or petals in a floral bud, to members of the same whorl. 1. Valvate: members of the whorl have touching, or nearly touching, margins that do not overlap. 2. Twisted (contorted): one margin of a member overlaps with the next member. This twisting may be clockwise or anticlockwise, as in the China rose. 3. Imbricate: one of the members is internal (i.e. both its margins are overlapped), one is external (none of its margins overlapped, whilst overlapping its neighbours), and the remaining members have one overlapped margin and one overlapping margin. 4. Vexillary: 5 petals, the posterior one being the largest and almost covering the two lateral petals and the laterals nearly overlap the two anterior petals, which are the smallest. Found in all papilionaceous corollas. Nectaries Nectaries secrete a sugary fluid (the main sugars are sucrose, glucose and fructose). They may be floral or extrafloral (e.g. Passiflora has nectaries on petioles) and may be a glandular surface or a more specialised structure. Phloem and xylem contribute to nectar secretion – if phloem predominates then the nectar may be 50% sugar (up to 70% in horse chestnut, Aesculus hippocastanum), but if xylem dominates it may be only 8% sugar.. Floral nectaries may occur on sepals, petals, stamens, ovaries or the receptacle. Extrafloral nectaries may occur on stems, leaves, stipules and pedicels. The glandular tissue may be epidermal or several layers deep and has an external cuticle. Nectaries are metabolically very active and modify the phloem sugars enzymatically. Nectary cells may be photosynthetic or rich in starch – to add extra sugar to the sap before it is secreted. Nectar may cling to the nectary surface or drain into the floral tube or spur. In Lonicera (honeysuckle), the nectar-secreting cells are short hairs on part of the inner lining of the corolla tube. Nectar secretion increases as the flower is visited by pollinators and nectar is resorbed once pollination is accomplished. Nectar may also contain amino acids, salts and proteins (nectarins) and other organic substances including vitamins and lipids. The bracken fern (Pteridium aquilinum) has nectaries at the bases of its leaves. Extrafloral nectaries may serve to attract animals that will defend the plant, as in Acacia, which attracts the ant Pseudomyrmex.
Passion flower (Passiflora) and its pollen. K5 [C5 CF72] A5 G(3)
Surfaces of petals: the petal epidermal cells are important to the petals optical properties. Narcissus pseudonarcissus L. in visible light (left) and ultraviolet (right).
Arnica angustifolia Vahl in visible light (left) and ultraviolet (right) – showing a bulls eye pattern. Coreopsis sp. lia in visible light (left) and ultraviolet (right) – showing a bulls eye pattern.
Geranium sylvaticum L. in visible light (left) and ultraviolet (right) – showing a bulls eye pattern. Oenothera biennis L. in visible light (left) and ultraviolet (right) – showing a bulls eye pattern. Potentilla anserina L. in visible light (left) and ultraviolet (right) – showing a bulls eye pattern.
Stamen Consists of an anther divided into pollen sacs (microsporangia) and borne on a thin single-veined stalk (filament). Each pollen sac includes wall layers and a locule in which microspores are produced. Most angiosperms have tetrasporangiate anthers – two locules in each of the two lobes. Some have bisporangiate anthers – one locule in each lobe. These partitions may break down when the anthers dehisce. Some have more primitive three-veined leaflike stamens with microsporangia on their adaxial surface (~95% have single-veined anthers). Filament: parenchyma around a vascular bundle which may be amphicribral (phloem surrounding xylem on both sides). Cutinised epidermis may bear trichomes and stomata may be present on both anther and filament. The vascular bundle ends blindly either in the anther base or in the connective tissue between the two anther halves. Dehiscence (spontaneous opening): the anther often ruptures (longitudinally, e.g. cotton, or transversely, e.g. basil) to form a slitlike opening or pore (stomium) or by a number of pores, e.g. potato, or by valves (valvular) e.g. bay leaf. The anther subepidermal layer (endothecium) bears strips of secondary wall thickenings which promote differential shrinking when the anther dries. Gynoecium The Carpel A flower may have one or more carpels which may be free (apocarpous) or united (syncarpous). Pistil: a single carpel in an apocarpous gynoecium (simple pistil) or an entire syncarpous gynoecium (compound pistil). The carpel is a folded modified leaf with its adaxial surfaces enclosed and the margins more or less completely united and more or less reduced, resulting in a unilocular gynoecium. Conduplicate folding: the margins remain flat. In some the margins exhibit involution (curling into the carpel space such that the suture is lined by the abaxial surfaces, resulting in a bilocular or multilocular gynoecium. Style: the commonly sterile upper part of an apocarpous gynoecium or of the entire syncarpous gynoecium. The ovary is the lower fertile part in such carpels. A sessile stigma occurs when no style is present. In some more primitive carpels, the carpels are folded styleless structures with stigmatic tissue covering the unsealed margins. Ovary: contains the ovary wall within, one or more locules and partitions between multiple locules. Ovules are borne on part of the adaxial (inner) side. An ovule-bearing region is called a placenta. Each carpel has two placentae. Marginal placentation: placentae close to the margins. Laminar placentation: placentae distant from the margins. Marginally joined carpels have placentae on the ovary wall (parietal placentation). In involuted bi- /multilocular carpels, the placentae occur in the centre of the ovary where the margins meet (axile placentation). The margins may disappear, resulting in a free central placentation. The placenta in a unilocular ovary may occur at the base (basal placentation). Epigynous flowers: the ovary is embedded in extracarpellary tissue derived from the receptacle or from the floral tube (fused bases of sepals, petals and stamens). Perigynous flowers: the gynooecium is enclosed in a cup of extracarpellary tissue but not joined to it.
Stamen of Prunus (A) and its parts: cross sections of anther (B), vascular bundle of filament (C), anther wall (D,E); and endothelium in face and sectional views of the secondary wall (F). 10 m Hypogynous flower (hypogyny): the ovary occupies the highest position on the receptacle (thalamus, pedicel). Attachment of the anther to the filament 1. Basifixed or innate: the filament is attached to the base of the anther, e.g. water lily, radish, sedge, mustard. 2. Adnate: the filament runs up the whole length of the anther, e.g. Magnolia. 3. Dorsified: the filament is attached to the back of the anther, e.g. passion-flower. 4. Versatile: the filament is attached to one point on the back of the anther, such that the anther can swing freely, e.g. grasses, palms. Adhesion (adnate or adherent stamens) 1. Epipetalous stamens: attached to the corolla by their filaments, e.g. sunflower, potato. 2. Epiphyllous stamens: attached to the perianth, e.g. Liliaceae. 3. Gynandrous stamens: attached to the carpels by their anthers, e.g. orchids. Cohesion (connate or coherent stamens) 1. Monaldelphous stamens: filaments united into a single staminal tube, anthers free, e.g. cotton. 2. Diadelphous stamens: filaments united into two bundles, anthers free, e.g. pea, bean (9 + 1 stamens). 3. Polyadelphous stamens: filaments united into more than two bundles, anthers free, e.g. lemon (Citrus). 4. Syngenesious stamens: anthers united, filaments free, e.g. sunflower, marigold (Compositae). 5.Synandrous stamens: both filaments and stamens united, e.g. Cucurbitaceae: wax gourd (Benincasa).
The ovary wall is mostly parenchymatous. The carpel usually has three veins – one median (dorsal) and two lateral (ventral). The lateral bundles especially supply the ovules. In some taxa the gynoecium may have sclerified tissue and accumulate tannins and other substances for protection. The outer epidermis is cuticularised and may have stomata. Style and Stigma Style: an upward prolongation of the carpel. Syncarpous gynoecia may have a single style derived from all the carpels. If incompletely united, the style may be united at the base and multiple at the top or there be as many stylar branches (stylodes) as carpels. Styles and stylodes may be solid or contain a central canal. Receptive stigmas may be covered with secretion of lipids and phenolic compounds (wet stigmas). Wet stigmas are glandular. The stigma epidermis often possesses papillae, short hairs or long branched hairs. The papillae may be covered by a protein pellicle. The pollen transmitting tissue connects the stigma with the ovule (lines in the canal in hollow types). Microsporangium and microspores The microsporangium contains sporogenous tissue which gives rise to microsopres (pollen grains). 1) Meristematic lobe – periclinal divisions produce the first layer (archesporial layer) beneath the protoderm. 2) The inner derivatives of the archesporial layer become the primary sporogenous cells and the outer derivatives from the primary parietal layer. 3) The primary parietal layer divides to form two secondary parietal layers and the outermost divides again to give three parietal layers. This produces: epidermis, future endothecium, middle layer and the innermost tapetum. Sometimes both secondary parietal layers divide to form two middle layers. In monocotyledons, the inner secondary parietal layer divides to form the middle layer and tapetum, whilst the outer differentiates into the endothecium. 4) The primary sporogenous cells either enlarge and differentiate into spore mother cells (SMCs) or divide to produce the SMCs. The SMCs are microsporocytes – cells that that produce haploid microspores by meiosis. Initially the SMCs are connected by plasmodesmata, then the original walls disintegrate and are replaced by callose and then the microsporocytes round up. Wide (1.5 m) cytoplasmic bridges connect them, where the plasmodesmata were. The microsporocytes form a coenocyte which enables their development to be synchronised. These connections disappear before meiosis two, leaving isolated tetrads (tetrahedral or tetragonal) of microspores. Tapetal cells (derived from both the primary parietal layer and from connective tissue) may become multinucleate or polyploid and probably nourish the pollen grains. In some the tapetum is a secretory (glandular) cell layer, in others it is amoeboid or plasmodial. After meiosis the tapetum breaks down and its remnants (tryphine) coat the pollen grains in an external lipid-rich coat. In the amoeboid type, the walls lyses and the intact protoplasts intrude among the pollen grains (and may fuse along the locule periphery to form a periplasmodium). Before anthesis, the plasmodium dehydrates and is deposited as tryphine on to the surfaces of the pollen grains.
Lilium Arabidopsis thaliana Morning glory
cellulose thickening nuclei The Pollen Grain
The Pollen Grain Wall
The middle layer is crushed between the tapetum and endothecium and absorbed. The mature endothecium has strips and bands of wall thickenings as may inner connective tissue cells. These thickenings are absent from the stomium. Pollen Grain The pollen grain wall consists of an exine and an intine. The exine may be subdivided into an outer sexine and an inner nexine (two layers). The sexine is sculpted and attached to the nexine by struts (bacula or columellae) which may unite into an outer tectum (tectate exine), or remain free (pilate exine). The pollen tube usually emerges through thin-walled areas (pores) in the exine during germination. These thin-walled areas also allow the pollen grain to change in volume with changes in humidity. Porate pollen – rounded apertures; colpate pollen – slitlike apertures. Monocolpate pollen – one aperture in monocotyledons; tricolpate pollen – three apertures in most dicotyledons. The pollen wall contains sporopollenin (carotenoid and carotenoid ester polymers) which is resistant to chemicals, high temperatures and decay. Silicon is present in some dicot exines. Pollen walls develop whilst the tetrads are still enclosed in callose and the microspores are wall-less. Endoplasmic reticulum accumulates beneath sites of future apertures. Elsewhere the first wall (primexine) is secreted, made of cellulose. Rods (probacula, probably lipid and protein) traverse the primary wall radially and these form a protoexine network which incorporates protosporopollenin. A layered intine (pectin and cellulose) is deposited beneath the exine. Male Gametophyte and Gametogenesis Before the pollen is shed, mitosis and cytokinesis produce a vegetative and generative cell and a two-celled gametophyte. The generative cell (tube cell) divides by mitosis into two wall- less sperms either before shedding or after germination of the pollen grain. The wall between the two has plasmodesmata and may contain callose. The generative cell rounds up and becomes surrounded by the vegetative cell, sometimes the wall between the two disappears and only two plasmalemma separate them. The sperm are ellipsoidal wall-less cells with numerous microtubules parallel to the long axis. Plastids have been observed in the sperm cells of some species. Pollination The pollen tube penetrates a papilla or grows along the surface of a stigmatic hair, before reaching the transmitting tissue in the style. The pollen tube grows through the solid transmitting tissue, in between cells which may contain a pectic substance or a mucilaginous substance. In styles with canals, the pollen tube grows along the tissue lining the canal or deeper in the lining. In lily pistils, it has been shown that the transmitting tissue releases chemoattractants for the pollen tube, distally at first, progressing basipetally before the growing pollen tube and the cells lining the canal have transfer-cell like wall ingrowths. Pollen tube The pollen tube grows several mm per hour (in vitro). The growth zone is the tip most 3-5 m. The wall is cellulosic or -1,3-polyglucan and intine. The older parts of the pollen tube may be sealed off by plugs of callose as the vegetative cell nucleus and the sperm cells migrate down the tube.
Anthesis: the time of flower expansion from receptive stigma to fertilisation. Megasporogenesis Ovule The ovule develops from the placenta of the ovary and is the site of formation of megaspores and the embryo sac (female gametophyte). The ovule typically consists of: 1) nucellus – the central body with vegetative cells enclosing the sporogenous cells – encloses the thin-wall of the embryo sac; 2) one or two integuments (unitegmic and bitegmic ovules) – formed by periclinal divisions of the epidermis; 3) funiculus – the stalk connecting the ovule with the placenta; 4) chalaza – the region where the nucellus, integuments and funiculus merge. The first sporogenous cell is the archesporial cell. An opening (the micropyle) remains where the integument arches over the nucellus. One or both integuments may contribute to the micropyle. Vascular tissue extends from the placenta to the funiculus and the walls of the embryo sac are highly vascular. The epidermis (on the outer integuments and funiculus) bears a cuticle, and the inner integuments and nucellus may bear a cuticle (three cuticles in total). The nucellus may be resorbed and then the embryo sac contacts the inner epidermis of the inner integument which may develop into the integumentary tapetum or endothelium rich in endoplasmic reticulum. This presumably nourishes the embryo sac, though there are no plasmodesmata between them (and two cuticles separate them), though the embryo sac wall cells may have wall ingrowths. Megaspores Megaspores result from the meiotic divisions of the spore mother cell (megasporocyte). The archesporial cell may be the megasporocyte or might divide into a megasporocyte and a parietal cell. The megasporocyte undergoes two meiotic divisions to form a linear tetrad of haploid megaspores. Three megaspores degenerate, whilst the chalazal megaspore enlarges and divides mitotically. Callose temporarily appears in the walls of the megaspore preparing for division. Female Gametophyte The megaspore cell divides three times by mitosis, to give 8 nuclei and 7 discrete cells – three cells at the micropylar end (the egg apparatus – the egg and two synergids) and three antipodal cells at the other end. The large central cell contains two polar nuclei, which may fuse before fertilisation to form the secondary endosperm nucleus. There are many variations in the mode of gametophyte formation. The common type described here (the Polygonum type) is seen in Solanum, for example. At their micropylar end, the synergids have a filiform apparatus (elaborate system of wall ingrowths) and no cell wall at the chalazal end (the cell wall covers two-thirds of the cell, as is also the case in the egg).
Fertilisation The pollen tube grows in the transmitting tissue lining the ovary wall and the placenta (and sometimes the funiculus). The pollen tube enters the embryo sac via the micropyle (or sometimes through the chalazal tissue). If the nucellus persists it must be crossed, though sometimes a column of nucellar cells degenerate to form a passage for the pollen tube. In some species the pollen tube enters through the filiform apparatus of one of the synergids – this particular synergid partly degenerates before the pollen tube arrives (releases chemoattractant?). The second synergid usually degenerates later (or at the same time). The pollen tube bursts open inside the synergid, the vegetative cell nucleus and synergid nucleus degenerate. The synergid plasmalemma disappears. One of the sperms fertilises the central cell, the other the egg – double fertilisation. A tightly coiled sperm has been observed inside the egg. Triple fusion of the two polar cells and one of the sperm produces the primary endosperm nucleus. Zygote The cell wall at the chalazal end becomes completed and protein synthesis begins and starch accumulates in the plastids. The cell undergoes its first division. The Vascular System of the Flower Resembles that of the vegetative shoot, but the branching and joining is more irregular due to the short internodes. The sepals typically have as many traces as the foliage leaves. Each petal in a dicotyledon has one trace, each tepal in a monocotyledon has one to many. Sepals and petals have complex vascular systems, similar to those of foliage leaves. The anther usually has one trace in the filament and anther, the carpel has three traces. Branches from the carpellary bundles supply the ovules and extend into the style. Vascular bundles may fuse in united parts. Pollination modes of trees and shrubs Wind (anemophily): alder, ash, beech, birches, bog myrtle, elms, hazel, hornbeam, oaks, poplars, juniper, Scots pine and yew. Insects (entomophily): broom (large bees), butterfly bush (butterflies), box (bees and flies), buckthorn, cherries, crab apple, elder (esp. small flies), gorses, hawthorns, holly (honey bees), horse chestnut (bees), limes (bees), pear, privet, rhododendron, roses, sweet chestnut, whitebeams Maples: wind and small insects. Willows: insects and birds. (Zoophilous plants are those normally pollinated by animals).
Flower parts of Aquilegia. Longitudinal views of sepal (A), petal (B), stamen (C), and carpel (D,F), and cross section of carpel (E). F
The angiosperm reproductive cycle
Pollination modes Self pollination (autogamy): the stigma of a flower receives the pollen of the same plant. This mode is frequent, but not compulsory, in cultivated Grasses. It is compulsory for flowers that do not open (cleistogamous) such as the Violet, balsam (Impatiens), sundew (Drosera), wood-sorrel (Oxalis), sage (Salvia). In homogamy, the anthers and stigmas of a bisexual flower mature at the same time.. Crossed pollination: the stigma of a flower receives the pollen of another plant. Promotion of Cross Pollination Dioecism: male flowers and female flowers are on separate plants (dioecious species). Dichogamy: male and female organs mature at different times. The pollen is before released while the stigma is immature (protandry) or the stigma is receptive while stamens are still young (protogyny). Hercogamy: some structures prevent pollen from being transferred on stigma of the same flower (rostellum of the Orchis). Heterostyly: in Primula, flowers with high style and stamens situated on the base of the corolla must be pollinated by flowers with short style and stamens situated on the top of the corolla (dimorphic heterostyly). Self sterility: flowers can't be self pollinated because of dimorphism in pollen grains and stigma surfaces. Means of pollination are the wind (anemophily) or insects (entomophily), other animals, less often water. In the first case, flowers generally have a well developed and coloured perianth. In the second case, there is no perianth or it is reduced and uncoloured. Pollinator type Flower typeFlower colourFlower smell Reward BeetlesUpwards facing bowl Brown, whiteStrongPollen, nectar FliesUpwards facing bowl Pale, dullLittleNectar BeesOften asymmetric, strong, semi-closed Yellow, blueFairly strong Nectar Butterflies, moths Horizontal or hanging Red, yellow, blue (day); white or pale (night) BirdsHanging or tubular, copious nectar Vivid redAbsentNectar BatsLarge strong single flowers or bush-like Greenish, cream, purple Strong at night Nectar, pollen
Special pollination mechanisms Pollinia: in some plants (orchids and some milkweeds), the pollen cells remain united to form a mass (pollinium). Two stalks (caudicles) grow out of the base of the anther, each attached to the posterior, sterile part of the stigma (rostellum) at the base and to the pollinium at the apex. The rostellum develops sticky glands, fixing the caudicles to it. The pollen are transferred as a single mass. In many orchids the pollinia are transferred from one flower to another by insects. Some orchids mimic the female insect to lure a male which picks up the pollinia when he attempts to mate with the flower. Ophrys insectifera flowers (above and left) mimic female flies. When a male fly attempts to mate with the flower, he picks up the pollinia. Other orchids mimic wasps and bees. Right: the bee orchid (Ophrys apifera).
Fig trees (Ficus): bear fruitlike structures called syconia, which contain minute male and female flowers. A tiny female wasp enters the opening (ostiole) on the syconium and pollinates the flowers and deposits an egg in each short-stylar female flower (inside the ovary through the stylar canal). Long-styled female flowers develop as normal, following double fertilisation and each develops a seed. Short-style flowers can also develop seed bearing drupelets. The endosperm of infected flowers nourishes the developing insect. Male and female wasps emerge and mate inside the syconium. The females pack their pollen baskets (corbiculae) with pollen and then emerge from the syconium.
Hydrophily in Aquatic Plants Aquatic plants, especially those that are submerged rely on water to transport their pollen. These flowers are usually small and inconspicuous. In Vallisneria, the plants are dioecious and submerged. The male plant bears a large number of minute male flowers in a small spadix surrounded by a spathe and borne on a short stalk. The female plant bears solitary female flowers, each on a long stalk. This stalk elongates and lifts the female plants to the surface of the water. The spathe bursts, releasing male flowers from the spadix, whilst closed, and float on the surface of the water. The perianth expands to give them buoyancy. Some of the floating male flowers contact the female flowers and the anthers dehisce and transfer pollen to the trifid stigmas which close up. After pollination the stalk of the female flower coils up into a helix, pulling the female flower down into the water. The fruit develops and matures beneath the water. The Inflorescence The inflorescence is the reproductive shoot bearing one or more flowers. It may be terminal or axillary. Racemose inflorescences: the main inflorescence axis (monopodial axis with or without a terminal flower) usually does not terminate in a flower but continues to grow and put out flowers laterally in acropetal succession (the lower flowers are older and flowers open in a centripetal manner). 1. Raceme: elongated main axis, stalked flowers, with the lower (older) flowers having longer stalks. E.g. radish (Raphanus), mustard (Brassica). Compound raceme (panicle): the main axis has branches and the lateral branches bear branches and flowers. A. With the main axis elongated 2. Spike: similar to a raceme but with sessile flowers, e.g. amaranth (Amaranthus). 3. Spikelets: very small spikes with one flower or a few florets. The spikelets are arranged in a spike, raceme or panicle and may be sessile or stalked. Each spikelet bears two minute bracts (empty glumes) at its base and a third bract (flowering glume or lemma) higher up and opposite the lemma is a small (2-veined) bracteole (palea). Each flower of the spikelet remains enclosed by the lemma and palea). Flowers and glumes are arranged in two opposite rows. E.g Gramineae, including grasses, paddy, wheat, sugarcane, bamboo, etc. 4. Catkin: a spike with a long and pendulous axis which usually bears unisexual flowers only, e.g. birch (Betula) and oak (Quercus). 5. Spadix: a spike with a fleshy axis, enclosed by one or more bracts (spathes), which may be brightly coloured, e.g. aroids, banana (Musa) and palms. Found in monocots only. B. With the main axis shortened 6. Corymb: the lower flowers have much longer stalks, so that all flowers are on the same level, e.g. wallflower (Cheiranthus).
7. Umbel: a group of flowers borne at the tip, with pedicels more or less the same length, so that the flowers appear to spread out from a common point. A whorl of bracts forms an involucre, and each flower develops from the axil of a bract. In a compound umbel the umbel is branched and each branch bears a bunch of flowers, e.g. carrot. A simple umbel is unbranched, e.g. wild coriander (Eryngium). Umbels are characteristic of the Umbelliferae (coriander family). C. With the main axis flattened 8. Head or capitulum – the receptacle is almost flat and bears stalkless flowers (florets). The outer flowers are the oldest and open first. The whole resembles a flower. There are usually two kinds of floret: ray florets are marginal and strap-shaped and disc florets are central and tubular. One or more whorls of often green bracts form an involucre at the base. E.g. Compositae (sunflower, marigold), Acacia (gum tree) and the sensitive plant (Mimosa). Visiting insects can pollinate several flowers in a short space of time and with little effort. The receptacle may be folded inward to form a pear-shaped hollow hypanthodium with a narrow entrance and the flowers born on the inner wall of the cavity, e.g. Ficus (fig, banyan). Cymose Inflorescences The main axis and lateral branches end in a flower and are determinate and cease growing. The branching system is predominantly sympodial. The flowers may be stalked or sessile. Flower development is basipetal – the terminal flower is the older and the lateral flowers the younger. The flowers open in a centrifugal direction. 1. Uniparous or monochasial cyme Each flower is borne in the axil of the bract of the preceding flower. In a helicoid cyme the lateral branches develop on one side in a helix. In a a scorpioid (cincinnus or alternate- sided) cyme, the lateral branches develop alternately on either side, in a zig-zag pattern. In a monochasial cyme each lateral branch bears a single flower and all are borne on a more or less straight sympodial pseudo-axis (sympodial cyme). In this case a bract appears opposite to a flower, whilst in a racemose a bract appears beneath each flower. E.g. forget-me-not (Myosotis) – scorpioid. 2. Biparous or dichasial cyme Each unit of the sympodium bears two flowers – the main axis ends in a flower and produces two lateral branches at the same time, and each branch behaves similarly. E.g. jasmine. 3. Multiparous or polychasial cyme Each unit of the sympodium produces more than two flowers. Looks like an umbel, but the middle flower opens first, e.g. blood-flower (Asclepias). Thyrse inflorescence Sympodial sequences born on a monopodial axis, with the lateral branches not born in whorls. Verticillaster inflorescence Sympodial sequences born on a monopodial axis, with the lateral branches in whorls.
Cyathium inflorescence A cup-shaped involucre encloses a single female flower (reduced to a pistil) in the centre on a long stalk and a number of male flowers (each reduced to solitary stamens with a scaly bract at the base) on short stalks around the female flower. The flowers develop centrifugally, with the female flower maturing first. Found in Euphorbia (e.g. poinsetia, spurges and Jews slipper (Pedilanthus)). Diagrammatic representation of inflorescence types. a, b) raceme, c, d) spike, e) spadix, f) catkin, g) panicle, h, i) corymb, j) capitulum, k) hypanthodium, l-n) umbel, o) dichasial chyme, p) pleiochasial cyme, q) thryse, r) verticillaster, s-v) monochasial cymes (s, rhipidium; t, drepanium; u, cincinnus; v, bostryx).
Bracts Bracts are special leaves from the axils of which arise one or more flowers. 1. Bracteole: a small leafy or scaly bract on the pedicel. 2. Foliaceous (leafy) bracts. 3. Spathe: a large, often boat-shaped bract enclosing a cluster of flowers or an inflorescence (spadix), e.g. banana, palms, aroids, maize, cob. 4. Petaloid bracts: brightly coloured bracts, e.g. the red leaf-shaped bracts of poinsettia (Poinsettia). 5. Epicalyx: one or more of the whorls of bracteoles develop at the base of the calyx, e.g. cotton, strawberry (Fragaria). 6. Scaly bracteole: at the base of individual florets on a capitulum in Compositae, there is often a thin, membranous awl-shaped scaly bracteole. 7. Glumes: small, dry and scaly bracts found in the Gramineae spikelet (empty glumes, lemma and palea).