The Flower

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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.
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Idealised Flower
Asteraceae (Daisies and sunflowers) a
flowerlike inflorescence of florets.
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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, _at_ 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.
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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.
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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.
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Passion flower (Passiflora) and its pollen. ? K5
C5 CF72 A5 G(3)
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Surfaces of petals the petal epidermal cells are
important to the petals optical properties.
Narcissus pseudonarcissus L. in visible light
(left) and ultraviolet (right).
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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.
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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.
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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). Epigyn
ous 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.
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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).
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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. Tapeta
l 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.
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Anthers
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Lilium
Arabidopsis thaliana
Morning glory
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The Pollen Grain
nuclei
cellulose thickening
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The Pollen Grain Wall
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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 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.
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Anthesis the time of flower expansion from
receptive stigma to fertilisation. Megasporogenesi
s 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).
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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).
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F
Flower parts of Aquilegia. Longitudinal views of
sepal (A), petal (B), stamen (C), and carpel
(D,F), and cross section of carpel (E).
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The angiosperm reproductive cycle
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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
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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).
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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.
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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).
30
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 Sympo
dial sequences born on a monopodial axis, with
the lateral branches in whorls.
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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).
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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).