Topic 1
Flowers
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1.1 Flower shapes and
inflorescences
This section deals with the various shapes of flowers and the way these
flowers can be arranged as a group to form an inflorescence.
When a flower shape is discussed, it is the shape of the petals and/or sepals
(the perianth) which constitutes a flower shape. However, because there are
also many flowers which do not fit exactly into any one of the flower shapes,
it is quite acceptable to combine more than one shape name to describe a
specific flower shape.
If the flower does not have distinct sepals and petals (perhaps only one
whorl) then it is the shape of that petal like structure that is described.
Figure 1 - Principal organs of a ‘typical’ flower.
Flower shapes
As you are now aware, the arrangement and shape of the perianth
segments (usually the petals) within the one flower structure gives rise to a
range of different shapes. Knowing the flower shapes makes identification of
some plant species easier and can be used to classify plants into plant
families. For example, all flowers which are in family Fabaceae, sub - fam.
Faboideae, have a papilionate flower shape. Again, labiate flowers are
usually in family Lamiaceae.
The stalk of a single flower is known as the pedicel.
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Tubulate
Shaped like a tube, the sides are parallel or nearly so, eg
Cuphea ignea (cigar flower).
Salverform
Having a slender cylindrical lower part and opening out
flat at the mouth, eg Phlox spp, Plumbago spp. The sides
of the lower part of floral tube are nearly parallel.
Funnelform
Funnel - shaped; petal lobes may turn back at the top,
but not as flat as salverform flowers. The sides of the
floral tubes are not parallel gradually widening from
the base to the top of the flower, eg Petunia x hybrida,
Hibiscus rosa - sinensis.
Campanulate
Bell - shaped; base of flower is more rounded than
funnelform flowers. Flowers normally hang down, eg
Abutilon x hybridum (Chinese lantern).
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Rotate
Wheel - shaped with a short tube at base and
spreading at the mouth; petals free from each other,
eg Hibbertia spp. (guinea flower), Tibouchina spp,
Lagerstroemia indica.
Stellate
Star - shaped, petals usually free from each
other, usually 4, 5 or 6 petals, eg Hemerocallis
spp. (day lily), Trachelospermum jasminoides.
Labiate
Having one or more petals forming a lip, eg Prostanthera
spp, Salvia splendens. Also bilabiate – having two lips, eg
Westringia fruticosa. Flowers with labiate or bilabiate shape
are typical of members of family Lamiaceae.
Saccate
Irregularly shaped flower with lowest petal
spurred or modified into a rounded sac.
Corolla tube often with a sac - like bulge on
one side, eg Grevillea spp., Nemanthus spp.
Calceolate
Shaped like the toe of a slipper, eg Paphiopedilum spp.
(slipper orchid), Calceolaria. Flower with large bulge on
one side.
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Papilionate
Shaped like the wings of a butterfly, eg Lathyrus
(sweet pea), Wisteria; made up of three distinct
petal types:
1. standard
2. wings
3. keel.
Typical of members of family Fabaceae, sub - fam. Faboideae.
Crateriform
Flowers shaped like a shallow open bowl, saucer, or
crater of an extinct volcano, eg Papaver spp. (poppy),
roses (single).
Cyathiform
Cup - shaped, and open at the top; more curved at the
bottom than funnelform and deeper than crateriform;
petals not curving out as in campanulate flowers, eg
Cobaea scandens (cup and saucer vine), Tulipa spp.
Urceolate
Urn - shaped, narrow at the mouth and wider below, similar
to cyathiform but nipped in towards the top; these flowers
are often quite small, eg Erica carnea (spring heath),
Epacaris spp, Arbutus unedo, Pieris japonica spp.
Radiate
Petals spreading like many rays from a ‘centre’ of
reproductive structures; more petals present than
rotate or stellate flowers (eg Mesembryanthemum
(pigface) Lampranthus). This term is often
incorrectly used to describe the inflorescence of
members of family Asteraceae.
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Having showy stamens
With these types of flowers, we are describing what we see (which are the
stamens) rather than the shape of the perianth. It does not mean that these
flowers do not have all the parts of a ‘typical’ flower, it is just that it is the
stamens which are obvious and showy. There are two types of flowers with
showy stamens.
Conspicuously staminate
Flwers with many conspicuous, often brightly coloured
and showy stamens; sepals and petals present but
may be small and inconspicuous (eg Melaleuca,
Callistemon).
Apetalous staminate
Fowers with many conspicuous, often brightly
coloured and showy stamens; flowers have no petals
when the stamens are mature (eg Eucalyptus). The
petals have been modified into a protective structure
which is shed as the stamens expand and the flower
matures.
Inflorescences
An inflorescence is defined as a group of flowers borne on one stalk or stem.
This inflorescence stalk is called the peduncle. The arrangement of
individual flowers on a peduncle gives rise to several different
inflorescences.
Some inflorescences can be very difficult to determine—in fact, some may
be a combination of two inflorescence types. For example, the inflorescence
of Lantana species is an umbellate spike. You may find it easier to identify
some inflorescences by studying them before all the flowers are opened, or
if the flowers have opened, shake the inflorescence gently and turn it upside
down.
You must always locate the flowers that have opened first; that is, the oldest
flowers in the inflorescence. This will help to determine the inflorescence
type. You will note that the individual flowers on some inflorescence
diagrams are numbered. Number 1 = oldest flower, or flower formed and
opened first, 2 = second formed and opening flower, 3 = third formed and
opening flower—and so on to the youngest flower on the inflorescence.
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Types of inflorescence
There are three main types of inflorescence:

solitary

raceme - type, or indeterminate

cyme - type, or determinate.
Solitary
Solitary flower borne singly on a pedicel. They may be
formed as a terminal flower or singly in the leaf axils up a
stem, eg Papaver nudicaule (poppy), Westringa fruticosa.
Raceme type or indeterminate inflorescences
These are all characterised by having the oldest flower opening at the base
of the inflorescence while growth is still continuing longitudinally.
Raceme
A group of flowers attached to the peduncle with pedicels
of about the same length; the oldest flower is at the
bottom of the inflorescence and the youngest at the top,
eg Salvia splendens.
Spike
A racemose inflorescence with sessile flowers (without
a pedicel), eg Callistemon spp. (bottlebrush).
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Panicle
A branched racemose
inflorescence with each
whole branch being a
raceme. The side
branches are smaller
racemes. A panicle is a
compound raceme, eg
Yucca spp.
Corymb
A racemose inflorescence in which all the flowers
are ultimately borne at the same level because
the pedicels are of uneven lengths. The flowers
towards the bottom of the peduncle (that is, the
oldest flowers) have the longest pedicels, eg
Iberis spp. (candytuft), Spiraea spp.
Simple umbel
A racemose inflorescence in which all the
pedicels are equal in length and arise at one point
on the peduncle. The flowers are at the same
level, eg Hippeastrum.
Compound umbel
An inflorescence made up of small umbels
arranged in a larger umbel, the peduncles are
equal in length, eg fennel, parsley.
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Catkin
A pendulous racemose inflorescence modified
for wind pollination. It is a loose spike make up
of numerous sessile, usually unisexual flowers.
Female catkins usually have long hairy styles
and stigmas to enhance pollen interception.
Capitulum (Head)
A racemose inflorescence with sessile flowers on a flattened and expanded
peduncle, eg Zinnia elegans.
Each inflorescence consists of two floret types

ray florets: a ligulate floret around the edge of a capitulum, has a
conspicuous ‘strap’;

disc florets: tubular florets in the centre of a capitulum.
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Cyme type or determinate inflorescences
These are characterised by having an inflorescence in which the terminal
flower terminates the growth of that inflorescence and other flowers arise
laterally below it. The oldest flowers are nearest the apex, eg Ranunculus
spp. (buttercup), Tibouchina spp.
When the flower arise both sides below the terminal flower sides the
inflorescence is a dichasial cyme.
When the flowers arise from one side only below the terminal flower, the
inflorescence is a monochasialcyme.
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1.2 The ‘typical’ flower
Flowers come in all shapes, sizes and colours, yet at the same time they
have some features in common.
Figure 2 - A ‘typical’ flower
Arrangement of floral parts
Floral parts are inserted on the receptacle of each flower. They can either be
arranged in whorls or spirally on the receptacle.
Whorls
In most flowers the calyx, corolla,
androecium and gynoecium are
arranged in circles or whorls on the
axis of the flower, eg rose, Petunia spp,
Hibiscus spp.
Spirals
Some flowers have the calyx, corolla,
androecium and gynoecium attached
at different levels in an ascending
spiral, eg Magnolia spp, Ranunculus
spp.
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Functions of floral parts
If the flower part is not directly involved with the reproduction process then it
is referred to as a non - essential part of the flower.
The essential parts of a flower are those concerned directly with the
reproduction process. They include both the essential male parts of these
flower and the essential female parts of the flower.
Non - essential flower parts
Calyx
If you examine a flower from the base,
the first part you see is a group of small
leaf - like structures arranged in a circle.
Each one of these is called a sepal and
the whole group of sepals is called the
calyx.
Before a flower bud opens, it is enclosed and protected by the calyx. The
calyx then opens out to let the other parts of the flower emerge.
Corolla
The next whorl in from the calyx you can see is a
group of (usually) coloured petals. Each petal may
have different shapes. A group of petals is known as
the corolla.
Perianth
This word can be used in the following ways:

as a collective term for the calyx and
corolla;

if either the calyx or corolla is absent, the
remaining whorl is called the perianth;

if there are two whorls present but both
whorls are similar in shape, size, texture and/or colour they are both
referred to as the perianth.
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Receptacle (or torus)
The tip of a stalk from
which the flower parts
arise, the shape may
vary: from convex, to
flattened or concave.
Where the receptacle is
concave, the ‘arms’
which extend out from
the receptacle and hold
the sepals and petals is
called the hypanthium.
Pedicel
This supports the flower and is attached to the stem.
Peduncle
This is the stalk of a whole inflorescence.
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Essential flower parts - male
The collective term for the total of all stamens in the one flower is the
Androecium.
Stamen
This consists of the:

anther, which produces
pollen grains containing
the male reproductive
cells;

filament, the stalk which
support the anther.
This figure shows the possible
types of stamen structures and
variations:
(a) stamen structure;
(b–c) anther dehiscence;
(d) staminodes.
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Essential flower parts - female
This is the collective term for the female parts of the flower is the
Gynoecium. It may be composed of one or more pistils.
Where there is one loculus only and therefore one carpel, the whole
structure (ie stigma, style and ovary) may be referred to as a carpel.
Pistil
This is a structure composed
of a stigma, style and ovary.
Stigma: the receptive tip of
the pistil which receives
pollen and on which the
pollen grain germinates.
Style: the stalk - like structure
which supports the stigma. The germinating pollen grain tube which
develops and grows down through the style to the ovary.
Ovary: the swollen base of the pistil which contains the ovules. Inside the
ovary are found:

ovules: the structure containing the female reproductive cell, called
the egg or ovum;

loculus: the cavity or chamber within the ovary where the ovules are
located;

placenta: the tissue within the ovary to which the ovules are attached;

carpel: the region of the ovary including the carpel wall, loculus,
placenta(s) and ovule(s). An ovary may be composed of a single
carpel or two or more fused carpels.
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1.3 How do flowers differ from each
other
If you look at the many different kinds of flowers, you will notice that their
structure varies. The sepals, petals, stamens and pistils may or may not be
present, and may be arranged in a variety of different ways, or the flowers
themselves may be arranged in different positions on the plant.
Presence or absence of flower parts
Complete flower
This is a flower with all parts present, that is the calyx,
corolla, androecium and gynoecium present (eg
Petunia).
Incomplete flower
This is a flower in which one or more of the parts is
missing (eg spinach has a flower with no corolla).
Perfect flower
This is a flower with both male and female parts
present (eg Petunia); also called bisexual.
Imperfect flower
This is a flower with only female or male parts present. Imperfect flowers are
divided into two categories:
Staminate flowers have only the stamens present
(eg male pawpaw flowers).
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Pistillate or carpellate flowers have only the pistils
present (eg female pawpaw flower).
Monoecious plant
This has both staminate and pistillate flowers on the
same plant (eg maize, cucumber).
Dioecious plant
This has staminate and pistillate flowers on
separate plants (eg avocado, chinese gooseberry).
Union of flower parts
In a flower, members of one whorl may be fused to each other or to
members of another whorl. The following terms describe some of the
conditions which can occur.
Apopetalous
The petals are free from each other and other parts
of the flower (eg Boronia).
Connate
Members of a single whorl are joined or fused
together (eg connate petals in Petunia).
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Sympetalous
Sympetaly is a special case of connation where the
petals are attached to one another (eg Narcissus).
Synstemonous
Synstemony is a special case of connation where
the stamens are joined together (eg Hibiscus).
Adnate
Members of two different whorls are attached to
each other. Most often adnation occurs between
stamen and petal or petals and sepals (eg in the
snapdragon flower, the filament of each stamen is
attached to the corolla rather than to the receptacle).
Epipetalous
Epipetaly is a special case of adnation where the stamens
are attached to the corolla (eg Petunia).
Symmetry of flowers
Zygomorphic (irregular flower)
A flower which is irregular (or zygomorphic) is one in which the perianth can
be divided into two similar halves through one plane only (eg sweet pea).
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Actinomorphic (regular flower)
In a flower which is described as being regular (or actinomorphic) the
perianth can be divided into two similar halves through a number of planes.
This characteristic is also described as radial symmetry (eg Petunia).
Position of the ovary
The position of the ovary is an important point to consider if you are trying to
identify a particular plant. There are three main ovary positions, although
within these there are numerous variations within each group. These main
positions are:

Superior with floral parts either hypogynous or perigynous.
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
Inferior with floral parts epigynous.

Half inferior with floral parts perigynous.
Superior ovary with hypogynous floral parts
This is where the point of attachment of the ovary to the receptacle is at the
base of the ovary only. The sepals, petals and stamens arise from (or are
attached) beneath the ovary on the receptacle.
The flowers parts, other than the pistil, when arranged in this manner are
referred to as being hypogynous.
Superior ovary with perigynous floral parts
In some flowers the sepals, petals and stamens are attached to the margin
of a cup - shaped structure which surrounds the ovary. The ‘cup - like
structure’ may be either an extension of the receptacle, or hypanthium or
‘floral tube’ which is made up of the fusion of the lower portions of the
sepals, petals and stamens. The hypanthium is not fused at any point to the
ovary, therefore, the ovary is still a superior ovary. The flower parts other
than the pistil are referred to as being perigynous.
Inferior ovary with epigynous floral parts
This is where the ovary is fused to an extension of the receptacle or the
hypanthium below the point of attachment to the sepals, petals and stamens.
The ovary tissue is fused around the base of the pistil (that is fused to the
ovary tissue).
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The flower parts other than the pistil are referred to as being epigynous.
Half - inferior ovary with perigynous floral parts
This is where the ovary is partly fused to an extension of the receptacle or
the hypanthium so that a portion of the ovary is visibly free from it.
The flower parts other than the pistil referred to as being perigynous.
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1.4 Reproductive growth
In the life cycle of a ‘typical’ flowering plant, a seed is planted, it germinates
and grows into a plant which produces flowers. These flowers usually
contain stamens and pistils. Pollen is formed within the anthers of the
stamens and is transferred by the process of pollination to the stigma of the
pistil. Pollen tubes grow down the style and eventually penetrate the embryo
sacs. Here the male gamete (contained in the pollen) and the female gamete
(the egg which is formed within the embryo sac) join to form a zygote which
develops into the embryo.
The male and female gametes (or the sex cells) are formed by a type of cell
division called meiosis. This process halves their chromosome number so
that, when fertilisation occurs and the male and female gametes unite, the
gene - carrying chromosome sets combine to form a zygote. This is called
fertilisation. These two processes of meiosis and fertilisation form the basis
of the sexual reproductive cycle.
Growth from the zygote state into the mature plant is by cell division called
mitosis.
Pollination
Pollination is the transfer of pollen from the anther to the stigma of a flower.
Self - pollination is pollination of a stigma by pollen from the same flower or
another flower of the same plant.
Cross - pollination is the transfer of pollen from the anther of a flower of
one plant to the stigma of a flower of another plant of the same or related
species.
Figure 3
(a) Self - pollination
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Cross-pollination results in increased variability of the offspring, because two
individuals contribute to the genetic material of the resulting individual.
Increased variability allows for greater adaptability to new environments and
so affords an evolutionary advantage to a species. Cross-pollination in many
species results in the production of more seeds or greater vigour of the
offspring.
Many angiosperms have adaptations that prevent or reduce self - pollination.
Examples of these include:

stamens and pistils of the same flower maturing at different times;

stigma unreceptive to the flower’s own pollen;

stigma held up above the anthers in the flower so the pollen is placed
on the stigma by pollinating animals which have collected pollen from
a different plant;

individuals which are dioecious or monoecious.
Pollination agencies
The transfer of pollen from anther to stigma (pollination) may be carried out
by:

insects such as bees, wasps, butterflies, moths, beetles and flies

birds

small mammals such as bats. Animals pollinate flowers whilst feeding
on pollen, nectar secreted by nectaries or on the flower parts
themselves. Some pollen from the anther sticks to the animal and is
removed when it comes into contact with the sticky surface of the
style. Animals may be attracted to certain flowers by the scent, colour
or pattern of the flowers, which they learn are a source of food.

wind pollinates flowers that are usually small and inconspicuous, and
without bright colours or nectar. The perianth is often reduced or
lacking and stamens may protrude from the flower, while the stigmas
are often large and branched. The pollen grains are small, light and
dry and are commonly produced in larger quantities than in animal pollinated flowers. All these features serve to help pollination by wind.
Pollen germination
Once pollen grains are in contact with the stigma, they germinate to produce
a pollen tube which grows downwards through the style to the ovule. If the
generative cell has not already divided, it soon does, forming the two sperm
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(the male gametes).
Figure 4 - Growth of the pollen tube
The tube nucleus plus the two sperm cells move down the pollen tube which
passes through the micropyle in the integuments of the ovule and the two
sperm are released into the embryo sac.
Figure 5 - Release of the sperm into the embryo sac
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Fertilisation
Fertilisation is the union of the haploid (n) male and female gametes to form
a diploid (2n) cell called the zygote, which develops into a diploid individual
as a result of mitotic division.
In the embryo sac, one of the two sperm nuclei fuses with the egg nucleus to
form the zygote (2n). The second sperm nucleus fuses with both of the polar
nuclei of the central cell which forms the triploid (3n) primary endosperm
nucleus. Because both sperm nuclei fuse with nuclei of the other cells, this
process (which occurs in angiosperms only) is known as double fertilisation.
Figure 6 - Fertilisation
This occurs in the ovule just after fertilisation.
Note that the fertilisation process has to be repeated for every ovule, so for
flowers of watermelons and pumpkins (and every other plant which produces
masses of seeds) hundreds of separate pollen grains have to germinate and
grow down the style, one to each ovule.
Seed development
Following fertilisation, the zygote produced by fertilisation divides by mitosis
to produce the embryo. The primary endosperm nucleus begins to divide
and forms a multicellular tissue, the endosperm. The endosperm grows by
means of food supplied by the parent plant and it in turn nourishes the
embryo. The embryo continues to grow until all the endosperm is absorbed.
In some seeds, the nucellus develops into a food - storing tissue known as
the perisperm.
As the embryo matures it develops one or two cotyledons (seed leaves)
which accumulate most of the food from the endosperm. Above the node
between the cotyledons is a stem, called the epicotyl, which may bear young
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leaves. These young leaves together with the epicotyl make up the plumule,
which will form the shoot of the plant after germination. The basal end of the
embryo is called the radicle, which develops into the primary root after
germination. The region between the radicle and the cotyledons is the
embryonic stem or hypocotyl.
The seed is surrounded by the seed coat or testa which develops from the
integuments of the ovule. The micropyle is often visible on the seed coat as
a small pore and is commonly associated will a scar called the hilum, which
remains after the seed separates from the funiculus.
As the seed ripens, the embryo gradually passes into a dormant state in
which it remains until it germinates.
Development of fruit
These changes which mark the transition of the flower into a young fruit are
called fruit set. The capacity of the flower to set fruit depends in many
instances on the receptivity of the female parts to the pollen.
The whole process of growth is summarised in the figure below which shows
the basic life cycle of an angiosperm.
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Figure 7 - Basic life cycle of an angiosperm
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