Structure of a Dicotyledonous Seed
Structure of a Monocotyledonous
Seed
Semi-Technical Description of a
Flowering Plant
Important Families of Flowering
Plants
Introduction
- Morphology
refers to the study of the form and structure of organisms, including
plants. In flowering plants, several organs such as roots, stems, leaves,
flowers, fruits, and seeds play vital roles.
- Flowering
plants (angiosperms) are the most diverse group of plants and show
significant variation in their morphological structure.
Parts of a flowering plant
The Root
The root
is the underground part of the plant that absorbs nutrients and provides
anchorage. Roots can be classified based on their origin and structure.
Types of
Roots
- Taproot
System:
- Found
in dicotyledonous plants (e.g., mustard).
- The
primary root grows directly from the radicle and forms lateral roots.
- Taproots
penetrate deep into the soil.
- Fibrous
Root System:
- Found
in monocots (e.g., wheat).
- The
primary root is short-lived and is replaced by a mass of thin roots from
the stem base.
- Adventitious
Roots:
- Found
in plants like grass and banyan trees.
- Roots
arise from plant parts other than the radicle (e.g., stems or leaves).
Functions
of Roots
- Absorption
of water and minerals.
- Anchoring
the plant in the soil.
- Storing
food (e.g., carrots and sweet potatoes).
- Producing
growth regulators like auxins.
Regions
of the Root
- Root
Cap: Protects the root tip as it moves through
the soil.
- Region
of Meristematic Activity: Contains small cells that
actively divide.
- Region
of Elongation: Cells here elongate and contribute to root
growth.
- Region
of Maturation: Cells differentiate into specialized
structures. Root hairs arise here, which help absorb water and minerals.
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The regions of the root-tip |
The Stem
The stem
is the ascending part of the plant axis that bears leaves, branches, and
flowers. It supports the aerial parts and helps transport water, minerals, and
food.
Characteristics
of Stems
- The
stem arises from the plumule of the embryo.
- It
is differentiated into nodes (where leaves arise) and internodes
(the space between two nodes).
- Buds are
present at the tip (apical bud) or at nodes (axillary buds).
Functions
of Stems
- Spreading
leaves for photosynthesis.
- Supporting
flowers and fruits.
- Conducting
water, minerals, and food.
- In
some plants, stems are modified for functions like storage (e.g., potato),
support (e.g., tendrils in grapes), and vegetative propagation (e.g.,
mint).
The Leaf
Leaves
are the main photosynthetic organs of plants. They are lateral structures
attached to the stem.
Parts of
a Leaf
- Leaf
Base: The point of attachment to the stem.
- Petiole:
The stalk that attaches the leaf blade to the stem.
- Lamina (Leaf Blade): The broad, flat part of the leaf containing veins and veinlets. Veins are important for the transport of nutrients and water.
Types of
Leaves
- Simple
Leaf: A single undivided blade (e.g., mango).
- Compound
Leaf: The blade is divided into leaflets.
- Pinnately
Compound: Leaflets arranged on both sides of a
central axis (rachis) (e.g., neem).
- Palmately
Compound: Leaflets attached at a single point (e.g.,
silk cotton).
Phyllotaxy
(Arrangement of Leaves)
- Alternate: Single
leaf arises at each node (e.g., sunflower).
- Opposite:
Pair of leaves arises at each node (e.g., guava).
- Whorled:
More than two leaves arise from the node (e.g., Alstonia).
Venation
(Arrangement of Veins)
- Reticulate
Venation: Network of veins, common in dicots (e.g.,
peepal).
- Parallel
Venation: Veins run parallel, common in monocots
(e.g., grass).
The
Inflorescence
Inflorescence
is the arrangement of flowers on the floral axis. There are two main types:
- Racemose:
The main axis continues to grow, and flowers are arranged in acropetal
succession (e.g., mustard).
- Cymose:
The main axis terminates in a flower, and subsequent flowers arise in a
basipetal order (e.g., jasmine).
The
Flower
Flowers
are the reproductive structures found in angiosperms (flowering plants). They
are critical to plant reproduction, playing a key role in pollination,
fertilization, and the production of seeds. Below is a detailed breakdown of
the various parts of a typical flower.
1. Structure of a Flower
A
flower is a modified shoot where the shoot apical meristem transforms into a
floral meristem. Flowers can be either unisexual (containing either male or
female reproductive organs) or bisexual (containing both male and female
reproductive organs). A typical flower is made up of four main parts called whorls:
- Calyx
(the outermost whorl)
- Corolla
(the second whorl)
- Androecium
(the male reproductive organ)
- Gynoecium
(the female reproductive organ) Parts of a flower
2. Detailed Components of a
Flower
2.1.
The Calyx
- The
calyx forms the outermost whorl of a flower and is composed of units
called sepals.
- Sepals
are usually green and leaf-like, offering protection to the developing
bud.
- Types
of Calyx:
- Gamosepalous
– Sepals are fused together.
- Polysepalous
– Sepals are free from each other.
The primary function of the calyx is to protect the developing flower in its bud stage and sometimes support the petals when in bloom.
2.2.
The Corolla
- The
corolla is the second whorl of the flower and consists of petals,
which are often brightly colored.
- The
petals are primarily responsible for attracting pollinators such as
insects and birds to the flower.
- Types
of Corolla:
- Gamopetalous
– Petals are fused together.
- Polypetalous
– Petals are free from each other.
Aestivation
is the mode in which sepals or petals are arranged in a bud. Types of
aestivation include:
- Valvate
– Sepals or petals touch at the edges but do not overlap.
- Twisted
– Each sepal or petal overlaps the next.
- Imbricate
– Sepals or petals overlap each other but not in any specific direction.
- Vexillary
– A specific type of arrangement found in pea and bean flowers. Types of aestivation in corolla : (a) Valvate (b) Twisted (c) Imbricate (d) Vexillary
2.3.
The Androecium (Male Reproductive Organ)
- The
androecium consists of stamens, which are the male reproductive
organs of the flower.
- Each
stamen has two main parts:
- Anther
– A structure that produces pollen grains.
- Filament
– A stalk that supports the anther.
The
stamen can either be free or fused with each other or with other floral parts.
Examples include:
- Epipetalous
– Stamens are attached to petals, as seen in brinjal.
- Monadelphous
– Stamens are fused into one bundle.
- Diadelphous
– Stamens are fused into two bundles, as in pea flowers.
- Polyadelphous
– Stamens are fused into more than two bundles.
2.4.
The Gynoecium (Female Reproductive Organ)
- The
gynoecium, also called the pistil, is composed of one or
more carpels, which are the female reproductive parts of
the flower.
- Each
carpel consists of:
- Stigma
– The sticky surface where pollen grains land.
- Style
– A slender stalk that connects the stigma to the ovary.
- Ovary
– The basal portion of the carpel that contains one or more ovules. After
fertilization, the ovary matures into a fruit, and the ovules develop
into seeds.
Placentation
refers to the arrangement of ovules within the ovary. Types of placentation
include:
- Marginal
– Ovules are arranged along the edge of the ovary, as in peas.
- Axile
– Ovules are attached to the central axis in a multilocular ovary, as in
tomatoes.
- Parietal
– Ovules are attached to the inner walls of the ovary.
- Free-central
– Ovules are attached to the central axis, but the ovary is unilocular.
- Basal
– A single ovule is attached to the base of the ovary. placentation :(a) Marginal(b) Axile(c) Parietal(d) Free central(e) Basal
3. Symmetry of Flowers
Flowers
can have different types of symmetry:
- Actinomorphic
(Radial Symmetry): Flowers that can be divided
into two equal halves along multiple planes, such as in mustard and
datura.
- Zygomorphic
(Bilateral Symmetry): Flowers that can
be divided into two equal halves only along one plane, such as in peas and
beans.
- Asymmetrical
Flowers: Flowers that cannot be divided
into equal halves by any plane.
4. Types of Flowers Based on
Ovary Position
The
position of floral parts concerning the ovary can be categorized as:
- Hypogynous:
The ovary is situated above the other floral parts, and the flower is
called superior. Example: Mustard, china rose.
- Perigynous:
The ovary is positioned in the middle, and floral parts arise from the rim
of the thalamus at the same level. The ovary is called half-inferior.
Example: Rose, plum.
- Epigynous:
The ovary is situated below the other floral parts, and the flower is
called inferior. Example: Guava, cucumber. Position of floral parts on thalamus : (a) Hypogynous (b) and (c)Perigynous (d) Epigynous
5. Types of Flowers Based on
Sexuality
- Bisexual
Flowers: Flowers containing both stamens
(male part) and carpels (female part), such as in lilies.
- Unisexual
Flowers: Flowers that contain either
stamens or carpels, such as in papaya and cucumber.
6. Floral Diagrams and Floral
Formulae
Floral
diagrams visually represent the structure and arrangement of different floral
parts. Floral formulae are symbolic representations that describe the number
and fusion of floral parts.
A
floral formula includes symbols representing:
- K
for calyx (sepals)
- C
for corolla (petals)
- A
for androecium (stamens)
- G
for gynoecium (carpels)
- Additional
symbols include:
- ⊕
for actinomorphic flowers
- ♀
for female parts
- ♂ for male parts
The Fruit
- A
fruit develops from the ovary after fertilization.
- If
fertilization doesn't occur, some fruits can still develop through parthenocarpy
(e.g., banana).
- Fruits
may be classified as dry or fleshy. For example, mango is a fleshy fruit
with a differentiated pericarp (outer epicarp, middle mesocarp, and inner
endocarp), while coconut has a fibrous mesocarp. Parts of a fruit : (a) Mango (b) Coconut
The Seed
After
fertilization, the ovules in a plant develop into seeds. A seed serves as a
crucial reproductive unit and is composed of two primary parts: the seed
coat and the embryo. The embryo contains the radicle, embryonal axis,
and one or two cotyledons. Monocots like wheat and maize have one
cotyledon, while dicots such as gram and pea have two.
Structure of a Dicotyledonous Seed
In dicot
seeds, the outermost layer is the seed coat, which is composed of two
layers:
- Testa:
The outer layer.
- Tegmen:
The inner layer.
On the
seed coat, there is a scar called the hilum, marking the point where the
seed was attached to the fruit. Just above the hilum is a tiny pore known as
the micropyle, which allows water to enter the seed during germination.
Within
the seed coat lies the embryo, consisting of:
- Embryonal
axis: The central part.
- Cotyledons:
Two fleshy seed leaves that store food for the developing embryo.
The
embryo is divided into two important parts:
- Radicle:
This is the embryonic root that will develop into the plant's root system.
- Plumule:
This is the embryonic shoot that will eventually grow into the plant’s
stem and leaves.
Some seeds, such as castor seeds, contain an additional food-storing tissue known as the endosperm, formed through a process called double fertilization. These seeds are termed endospermic seeds. However, in plants like beans, gram, and peas, the endosperm is absent at maturity, and these seeds are called non-endospermic seeds.
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Structure of a Monocotyledonous Seed
Monocot
seeds, like those found in maize and cereals, are typically endospermic,
meaning they contain an endosperm that stores nutrients. However, some
monocots, like orchids, are non-endospermic, lacking an endosperm at
maturity.
In seeds
such as maize, the seed coat is thin and often fused with the fruit
wall, forming a protective covering. Inside, the endosperm is bulky and
stores essential nutrients for the growing embryo.
Between
the endosperm and the embryo is a protein-rich aleurone layer, which
plays a role in the digestion of stored food during germination.
The
embryo itself consists of:
- Scutellum: A
large, shield-shaped cotyledon found in monocots.
- Plumule:
The embryonic shoot.
- Radicle:
The embryonic root.
Both the plumule and the radicle are enclosed in protective sheaths. The coleoptile is the sheath that covers the plumule, while the coleorhiza encloses the radicle, safeguarding them during early stages of germination.
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Semi-Technical
Description of a Flowering Plant
- The
scientific description of a flowering plant starts with its vegetative
characters (roots, stems, leaves), followed by its floral characters
(inflorescence, flower parts).
- For
example, the floral formula uses symbols to represent different
aspects of the flower, such as:
- K
for calyx, C for corolla, A for androecium, and G
for gynoecium.
- Fusion
of floral parts is indicated by enclosing numbers in brackets. Floral diagram withfloral formula
Important
Families of Flowering Plants
Family: Solanaceae (Potato Family)
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| (c) L.S. of flower (d) Stamens (e) Carpel (f) Floral diagram |
- Vegetative
Characters:
- Herbs,
shrubs, or small trees.
- Stem:
Aerial, branched, cylindrical.
- Leaves:
Simple or compound with reticulate venation.
- Floral
Characters:
- Inflorescence:
Solitary, cymose.
- Flower:
Bisexual, actinomorphic.
- Calyx:
Sepals united, persistent.
- Corolla:
Petals united.
- Androecium:
Stamens epipetalous.
- Gynoecium:
Bicarpellary, ovary superior, placenta swollen with many ovules.
- Economic
Importance:
- Sources
of food (tomato, brinjal, potato), medicine (belladonna), and fumigation
(tobacco).
The study
of morphology in flowering plants is crucial for understanding their
classification, functions, and adaptation mechanisms. The diverse forms and
structures allow plants to thrive in different environments, fulfilling various
ecological roles. A thorough knowledge of plant morphology provides the basis
for botanical studies, including plant identification and taxonomy.
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