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
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 flower, @ 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.
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 boatshaped 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 petal’s optical
properties.
Narcissus pseudonarcissus L. in visible light (left) and ultraviolet (right).
Arnica angustifolia Vahl in visible light (left) and ultraviolet (right) – showing a ‘bull’s’ eye pattern.
Coreopsis sp. lia in visible light (left) and ultraviolet (right) – showing a ‘bull’s’ eye pattern.
Geranium sylvaticum L. in visible light (left) and ultraviolet (right) – showing a ‘bull’s’ eye pattern.
Oenothera biennis L. in visible light (left) and ultraviolet (right) – showing a ‘bull’s’ eye pattern.
Potentilla anserina L. in visible light (left) and ultraviolet (right) – showing a ‘bull’s’ 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.
Hypogynous flower (hypogyny): the ovary occupies the highest position on the receptacle
(thalamus, pedicel) .
10 mm
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).
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 mm) 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.
Anthers
Lilium
Arabidopsis thaliana
Morning glory
The Pollen Grain
nuclei
cellulose thickening
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 wallless 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 mm.
The wall is cellulosic or b-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).
F
Flower parts of Aquilegia. Longitudinal views of sepal (A), petal (B), stamen (C),
and carpel (D,F), and cross section of carpel (E).
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 type
Flower colour
Flower
smell
Reward
Beetles
Upwards facing
bowl
Brown, white
Strong
Pollen, nectar
Flies
Upwards facing
bowl
Pale, dull
Little
Nectar
Bees
Often asymmetric,
strong, semi-closed
Yellow, blue
Fairly
strong
Nectar
Butterflies,
moths
Horizontal or
hanging
Red, yellow, blue
(day); white or pale
(night)
Birds
Hanging or tubular,
copious nectar
Vivid red
Absent
Nectar
Bats
Large 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 alternatesided) 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 Jew’s
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).