5. MORPHOLOGY OF FLOWERING PLANTS
THE FLOWER AND THE INFLORESCENCE
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A flower is a modified shoot wherein the
shoot apical meristem changes to floral meristem.
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Internodes do not elongate and the axis
gets condensed.
- The apex produces different kinds of
floral appendages laterally at successive nodes instead of leaves.
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When a shoot tip transforms into a
flower, it is solitary.
- The arrangement of cluster of flowers on the floral axis is called inflorescence.
- Based on whether the apex gets converted
into a flower or continues to grow, inflorescences are 2 types: Racemose and
Cymose.
o Racemose: In this, the main axis continues to grow.
Flowers are borne laterally in an acropetal succession.
o Cymose: In this, main axis terminates in a
flower, hence is limited in growth. Flowers are borne in a basipetal order.
THE
FLOWER
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It is the reproductive unit in the
angiosperms.
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It is meant for sexual reproduction.
- A flower has a stalk (pedicel). Its swollen end is called thalamus
(receptacle).
- Reduced leaf found at the base of the
pedicel is called bracts. Flowers with bracts are called bracteate and
those without bracts, ebracteate.
- A typical flower has 4 kinds of whorls arranged on thalamus- calyx,
corolla, androecium & gynoecium.
- Calyx & corolla are accessory organs, while androecium and
gynoecium are reproductive organs.
- In flowers like lily, the calyx and corolla are not distinct. It is
termed as perianth.
- When a flower has both androecium and gynoecium,
it is bisexual. A flower having either only androecium or only gynoecium
is unisexual.
Based on symmetry, flowers are 3 types:
o Actinomorphic (radial symmetry): Here, a flower can be
divided into 2 equal radial halves in any radial plane passing through the
centre. E.g. mustard, datura, chilli.
o Zygomorphic (bilateral symmetry): Here, a flower can
be divided into two similar halves only in a particular vertical plane. E.g.
pea, gulmohur, bean, Cassia.
o Asymmetric (irregular): Here, a flower cannot be
divided into two similar halves by any vertical plane passing through the
centre. E.g. canna.
Based on number of floral appendages,
flowers are classified as follows:
o Trimerous: Floral appendages are multiple of 3.
o Tetramerous: Floral appendages are multiple of 4.
o Pentamerous: Floral appendages are multiple of 5.
Based on the position of calyx, corolla and androecium in respect
of the ovary on thalamus, the flowers are 3 types:
o Hypogynous: Here, gynoecium occupies the highest
position while other parts are situated below it. The ovary is superior. E.g.
mustard, China rose & brinjal.
o Perigynous: Here, gynoecium is situated in the centre
and other parts are located on the rim of the thalamus at the same level. Ovary
is half inferior. E.g. plum, rose, peach.
o Epigynous: Here, the margin of thalamus grows upward enclosing the ovary completely and getting fused with it. Other parts arise above the ovary. The ovary is inferior. E.g. Guava, cucumber, ray florets of sunflower.
Parts of a Flower
a) Calyx
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It is the outermost whorl of flower. It
is made of sepals.
- Generally, sepals are green, leaf like and protect the flower in
the bud stage.
- The calyx may be gamosepalous (sepals united) or polysepalous
(sepals free).
b) Corolla
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It is the whorl inner to calyx. It is
composed of petals.
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Petals have bright colour to attract
insects for pollination.
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Corolla may be gamopetalous (petals
united) or polypetalous (petals free).
- Shape and colour of corolla vary in
plants. Corolla may be tubular, bell-shaped, funnel-shaped or wheel-shaped.
Aestivation: It is the mode of arrangement of sepals and
petals in floral bud. It is many types:
o Valvate: Sepals or petals in
a whorl just touch one another at the margin, without
overlapping. E.g. Calotropis.
o Twisted: One margin of the appendage overlaps that
of the next one and so on. E.g. China rose, lady’s finger & cotton.
o Imbricate: Margins of sepals or
petals overlap one another but not in any particular direction. E.g. Cassia & gulmohur.
o Vexillary (papilionaceous): In this, there are five petals; the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel). E.g. pea & bean.
c) Androecium
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The male reproductive part composed of stamens.
- Each stamen represents the male
reproductive organ. It consists of a stalk (filament) and an anther.
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Each anther is usually bilobed.
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Each lobe has 2 chambers called
pollen-sacs.
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In pollen-sacs, pollen grains are produced.
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A sterile stamen is called staminode.
- When stamens are attached to petals, they
are epipetalous. E.g. brinjal. When stamens are attached to perianth
they are epiphyllous. E.g. lily.
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If the stamens are free, it is called polyandrous.
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If they are united, it is called synandrous.
It is many types:
o Monadelphous: Stamens are united into one bunch or one
bundle. E.g. China rose.
o Diadelphous: Stamens are united into two bundles.
E.g. pea.
o Polyadelphous: Stamens are united into more than two
bundles. E.g. citrus.
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There may be a variation in the length of
filaments within a flower. E.g. Salvia and mustard.
d) Gynoecium
(Pistil)
The female reproductive part made up of one or
more carpels.
A carpel has 3 parts:
o Stigma: It is the receptive surface for
pollen grains. It is usually at the tip of the style.
o Style: An elongated tube that connects ovary to
stigma.
o Ovary: It is the enlarged basal part on which the
style lies. Each ovary bears one or more ovules attached to a flattened,
cushion-like placenta.
Polycarpellary pistils (pistil with many carpels) are
2 types:
o Apocarpous: Carpels are free. E.g. lotus and rose.
o Syncarpous: Carpels are fused. E.g. mustard and
tomato.
Placentation: It is the arrangement of ovules on the
placenta within the ovary. It is many types:
o Marginal: Here, the placenta forms a ridge along
the ventral suture of the ovary and the ovules are borne on this ridge forming
two rows. E.g. pea.
o Axile: Here, the placenta is axial and the
ovules are attached to it in a multilocular ovary. E.g. China rose, tomato and
lemon.
o Parietal: Here, the ovules develop on the inner
wall of the ovary or on peripheral part. Ovary is one-chambered but it becomes
two-chambered due to the formation of the false septum. E.g. mustard and Argemone.
o Basal: Here, placenta develops at the base of
ovary and a single ovule is attached to it. E.g. sunflower, marigold.
o Free central: Here, ovules are borne on central axis
and septa are absent. E.g. Dianthus and Primrose.
After fertilisation, the ovules develop into seeds and the ovary matures into a fruit.
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