Chapter 5: Morphology of Flowering Plants
History of Plant Biology
Initially, biology was primarily descriptive, focusing on observing and documenting the diverse forms of life using naked eyes, magnifying lenses, and microscopes.
Descriptions included gross structural features, both external and internal, along with observable living phenomena.
Before experimental biology and physiology became established, naturalists focused on natural history.
Detailed descriptions were later utilized in reductionist biology, where living processes gained more attention than the description of life forms.
These descriptions became meaningful in framing research questions in physiology or evolutionary biology.
Katherine Esau (1898 – 1997)
Born in Ukraine in 1898.
Studied agriculture in Russia and Germany.
Received her doctorate in 1931 in the United States.
Early publications reported that the curly top virus spreads through a plant via the phloem tissue.
Authored "Plant Anatomy" (1954), which enhanced the understanding of plant structure and revived the discipline.
Authored "Anatomy of Seed Plants" (1960), referred to as "Webster’s of plant biology."
Elected to the National Academy of Sciences in 1957, becoming the sixth woman to receive that honor.
Received the National Medal of Science from President George Bush in 1989.
Peter Raven noted that she 'absolutely dominated' the field of plant biology even at the age of 99.
Introduction to Flowering Plants
Angiosperms exhibit a wide range of diversity in external structure or morphology.
They are characterized by the presence of roots, stems, leaves, flowers, and fruits.
Classification of plants is based on morphological and other characteristics.
Understanding higher plants requires knowledge of standard technical terms and definitions.
Variations in different plant parts are adaptations to their environment, such as adaptations to various habitats, for protection, climbing, and storage.
All weeds have roots, stems, and leaves, and may bear flowers and fruits.
The underground part of the flowering plant is the root system, while the portion above the ground forms the shoot system.
5.1 The Root
In the majority of dicotyledonous plants, the direct elongation of the radicle leads to the formation of the primary root, which grows inside the soil.
It bears lateral roots of several orders, referred to as secondary, tertiary, etc., roots.
The primary roots and their branches constitute the tap root system, as seen in the mustard plant.
In monocotyledonous plants, the primary root is short-lived and is replaced by a large number of roots.
These roots originate from the base of the stem and constitute the fibrous root system, as seen in the wheat plant.
In some plants, like grass, Monstera, and the banyan tree, roots arise from parts of the plant other than the radicle and are called adventitious roots.
The main functions of the root system are:
Absorption of water and minerals from the soil.
Providing a proper anchorage to the plant parts.
Storing reserve food material.
Synthesis of plant growth regulators.
5.1.1 Regions of the Root
The root is covered at the apex by a thimble-like structure called the root cap.
It protects the tender apex of the root as it makes its way through the soil.
A few millimeters above the root cap is the region of meristematic activity.
The cells of this region are very small, thin-walled, and with dense protoplasm.
They divide repeatedly.
The cells proximal to this region undergo rapid elongation and enlargement and are responsible for the growth of the root in length.
This region is called the region of elongation.
The cells of the elongation zone gradually differentiate and mature.
This zone, proximal to the region of elongation, is called the region of maturation.
From this region some of the epidermal cells form very fine and delicate, thread-like structures called root hairs.
These root hairs absorb water and minerals from the soil.
5.2 The Stem
The stem is the ascending part of the axis bearing branches, leaves, flowers, and fruits.
It develops from the plumule of the embryo of a germinating seed.
The stem bears nodes and internodes.
The region of the stem where leaves are born are called nodes, while internodes are the portions between two nodes.
The stem bears buds, which may be terminal or axillary.
The stem is generally green when young and later often becomes woody and dark brown.
The main function of the stem is spreading out branches bearing leaves, flowers, and fruits.
It conducts water, minerals, and photosynthates.
Some stems perform the function of storage of food, support, protection, and of vegetative propagation.
5.3 The Leaf
The leaf is a lateral, generally flattened structure borne on the stem.
It develops at the node and bears a bud in its axil.
The axillary bud later develops into a branch.
Leaves originate from shoot apical meristems and are arranged in an acropetal order.
They are the most important vegetative organs for photosynthesis.
A typical leaf consists of three main parts: leaf base, petiole, and lamina.
The leaf is attached to the stem by the leaf base and may bear two lateral small leaf-like structures called stipules.
In monocotyledons, the leaf base expands into a sheath covering the stem partially or wholly.
In some leguminous plants, the leaf base may become swollen, which is called the pulvinus.
The petiole helps hold the blade to the light.
Long, thin, flexible petioles allow leaf blades to flutter in the wind, thereby cooling the leaf and bringing fresh air to the leaf surface.
The lamina or the leaf blade is the green expanded part of the leaf with veins and veinlets.
There is, usually, a middle prominent vein, which is known as the midrib.
Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals, and food materials.
The shape, margin, apex, surface, and extent of incision of lamina varies in different leaves.
5.3.1 Venation
The arrangement of veins and the veinlets in the lamina of a leaf is termed as venation.
When the veinlets form a network, the venation is termed as reticulate.
When the veins run parallel to each other within a lamina, the venation is termed as parallel.
Leaves of dicotyledonous plants generally possess reticulate venation, while parallel venation is the characteristic of most monocotyledons.
5.3.2 Types of Leaves
A leaf is said to be simple when its lamina is entire or when incised, the incisions do not touch the midrib.
When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called compound.
A bud is present in the axil of petiole in both simple and compound leaves, but not in the axil of leaflets of the compound leaf.
The compound leaves may be of two types:
In a pinnately compound leaf, a number of leaflets are present on a common axis, the rachis, which represents the midrib of the leaf as in neem.
In palmately compound leaves, the leaflets are attached at a common point, i.e., at the tip of petiole, as in silk cotton.
5.3.3 Phyllotaxy
Phyllotaxy is the pattern of arrangement of leaves on the stem or branch.
This is usually of three types – alternate, opposite, and whorled.
In alternate type of phyllotaxy, a single leaf arises at each node in an alternate manner, as in china rose, mustard, and sunflower plants.
In opposite type, a pair of leaves arise at each node and lie opposite to each other as in Calotropis and guava plants.
If more than two leaves arise at a node and form a whorl, it is called whorled, as in Alstonia.
5.4 The Inflorescence
A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem.
Internodes do not elongate, and the axis gets condensed.
The apex produces different kinds of floral appendages laterally at successive nodes instead of leaves.
When a shoot tip transforms into a flower, it is always solitary.
The arrangement of flowers on the floral axis is termed as inflorescence.
Depending on whether the apex gets developed into a flower or continues to grow, two major types of inflorescences are defined – racemose and cymose.
In racemose type of inflorescences the main axis continues to grow, the flowers are borne laterally in an acropetal succession.
In cymose type of inflorescence the main axis terminates in a flower, hence is limited in growth. The flowers are borne in a basipetal order.
5.5 The Flower
The flower is the reproductive unit in the angiosperms.
It is meant for sexual reproduction.
A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called thalamus or receptacle.
These are calyx, corolla, androecium, and gynoecium.
Calyx and corolla are accessory organs, while androecium and gynoecium are reproductive organs.
In some flowers like lily, the calyx and corolla are not distinct and are termed as perianth.
When a flower has both androecium and gynoecium, it is bisexual.
A flower having either only stamens or only carpels is unisexual.
In symmetry, the flower may be actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry).
When a flower can be divided into two equal radial halves in any radial plane passing through the center, it is said to be actinomorphic, e.g., mustard, datura, chilli.
When it can be divided into two similar halves only in one particular vertical plane, it is zygomorphic, e.g., pea, gulmohur, bean, Cassia.
A flower is asymmetric (irregular) if it cannot be divided into two similar halves by any vertical plane passing through the center, as in canna.
A flower may be trimerous, tetramerous, or pentamerous when the floral appendages are in multiples of 3, 4, or 5, respectively.
Flowers with bracts-reduced leaf found at the base of the pedicel - are called bracteate and those without bracts, ebracteate.
Based on the position of calyx, corolla and androecium in respect of the ovary on thalamus, the flowers are described as hypogynous, perigynous and epigynous.
In the hypogynous flower the gynoecium occupies the highest position while the other parts are situated below it. The ovary in such flowers is said to be superior, e.g., mustard, china rose and brinjal.
If gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level, it is called perigynous. The ovary here is said to be half inferior, e.g., plum, rose, peach.
In epigynous flowers, the margin of thalamus grows upward enclosing the ovary completely and getting fused with it, the other parts of flower arise above the ovary. Hence, the ovary is said to be inferior as in flowers of guava and cucumber, and the ray florets of sunflower.
5.5.1 Parts of a Flower
Each flower normally has four floral whorls, viz., calyx, corolla, androecium, and gynoecium.
5.5.1.1 Calyx
The calyx is the outermost whorl of the flower and the members are called 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).
5.5.1.2 Corolla
Corolla is composed of petals.
Petals are usually brightly colored to attract insects for pollination.
Like calyx, corolla may also be gamopetalous (petals united) or polypetalous (petals free).
The shape and color of corolla vary greatly in plants.
Corolla may be tubular, bell-shaped, funnel-shaped, or wheel-shaped.
Aestivation: The mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl is known as aestivation.
The main types of aestivation are valvate, twisted, imbricate and vexillary.
When sepals or petals in a whorl just touch one another at the margin, without overlapping, as in Calotropis, it is said to be valvate.
If one margin of the appendage overlaps that of the next one and so on as in china rose, lady’s finger and cotton, it is called twisted.
If the margins of sepals or petals overlap one another but not in any particular direction as in Cassia and gulmohur, the aestivation is called imbricate.
In pea and bean flowers, there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is known as vexillary or papilionaceous.
5.5.1.3 Androecium
Androecium is composed of stamens.
Each stamen, which represents the male reproductive organ, consists of a stalk or a filament and an anther.
Each anther is usually bilobed, and each lobe has two chambers, the pollen-sacs.
The pollen grains are produced in pollen-sacs.
A sterile stamen is called a staminode.
Stamens of flower may be united with other members such as petals or among themselves.
When stamens are attached to the petals, they are epipetalous as in brinjal, or epiphyllous when attached to the perianth as in the flowers of lily.
The stamens in a flower may either remain free (polyandrous) or may be united in varying degrees.
The stamens may be united into one bunch or one bundle (monoadelphous) as in china rose, or two bundles (diadelphous) as in pea, or into more than two bundles (polyadelphous) as in citrus.
There may be a variation in the length of filaments within a flower, as in Salvia and mustard.
5.5.1.4 Gynoecium
Gynoecium is the female reproductive part of the flower and is made up of one or more carpels.
A carpel consists of three parts namely stigma, style and ovary.
Ovary is the enlarged basal part, on which lies the elongated tube, the style.
The style connects the ovary to the stigma.
The stigma is usually at the tip of the style and is the receptive surface for pollen grains.
Each ovary bears one or more ovules attached to a flattened, cushion-like placenta.
When more than one carpel is present, they may be free (as in lotus and rose) and are called apocarpous.
They are termed syncarpous when carpels are fused, as in mustard and tomato.
After fertilization, the ovules develop into seeds and the ovary matures into a fruit.
Placentation: The arrangement of ovules within the ovary is known as placentation.
The placentation is of different types namely, marginal, axile, parietal, basal, central and free central.
In marginal placentation the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows, as in pea.
When the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentation is said to be axile, as in china rose, tomato and lemon.
In parietal placentation, 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.
When the ovules are borne on central axis and septa are absent, as in Dianthus and Primrose the placentation is called free central.
In basal placentation, the placenta develops at the base of ovary and a single ovule is attached to it, as in sunflower, marigold.
5.6 The Fruit
The fruit is a characteristic feature of the flowering plants.
It is a mature or ripened ovary, developed after fertilization.
If a fruit is formed without fertilization of the ovary, it is called a parthenocarpic fruit.
Generally, the fruit consists of a wall or pericarp and seeds.
The pericarp may be dry or fleshy.
When pericarp is thick and fleshy, it is differentiated into the outer epicarp, the middle mesocarp and the inner endocarp.
In mango and coconut, the fruit is known as a drupe.
They develop from monocarpellary superior ovaries and are one seeded.
In mango the pericarp is well differentiated into an outer thin epicarp, a middle fleshy edible mesocarp and an inner stony hard endocarp.
In coconut which is also a drupe, the mesocarp is fibrous.
5.7 The Seed
The ovules after fertilization, develop into seeds.
A seed is made up of a seed coat and an embryo.
The embryo is made up of a radicle, an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).
5.7.1 Structure of a Dicotyledonous Seed
The outermost covering of a seed is the seed coat.
The seed coat has two layers, the outer testa and the inner tegmen.
The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit.
Above the hilum is a small pore called the micropyle.
Within the seed coat is the embryo, consisting of an embryonal axis and two cotyledons.
The cotyledons are often fleshy and full of reserve food materials.
At the two ends of the embryonal axis are present the radicle and the plumule.
In some seeds such as castor the endosperm formed as a result of double fertilization, is a food storing tissue and called endospermic seeds.
In plants such as bean, gram and pea, the endosperm is not present in mature seeds and such seeds are called non-endospermous.
5.7.2 Structure of Monocotyledonous Seed
Generally, monocotyledonous seeds are endospermic but some as in orchids are non-endospermic.
In the seeds of cereals such as maize the seed coat is membranous and generally fused with the fruit wall.
The endosperm is bulky and stores food.
The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer.
The embryo is small and situated in a groove at one end of the endosperm.
It consists of one large and shield shaped cotyledon known as scutellum and a short axis with a plumule and a radicle.
The plumule and radicle are enclosed in sheaths which are called coleoptile and coleorhiza respectively.
5.8 Semi-Technical Description of a Typical Flowering Plant
Various morphological features are used to describe a flowering plant.
The description has to be brief, in a simple and scientific language and presented in a proper sequence.
The plant is described beginning with its habit, vegetative characters – roots, stem and leaves and then floral characters inflorescence and flower parts.
After describing various parts of plant, a floral diagram and a floral formula are presented.
The floral formula is represented by some symbols.
In the floral formula, Br stands for bracteate, K stands for calyx, C for corolla, P for perianth, A for androecium, and G for Gynoecium. for superior ovary and for inferior ovary. for male, for female, for bisexual plants, for actinomorphic and % for zygomorphic nature of flower.
Fusion is indicated by enclosing the figure within bracket and adhesion by a line drawn above the symbols of the floral parts.
A floral diagram provides information about the number of parts of a flower, their arrangement and the relation they have with one another.
The position of the mother axis with respect to the flower is represented by a dot on the top of the floral diagram.
Calyx, corolla, androecium and gynoecium are drawn in successive whorls, calyx being the outermost and the gynoecium being in the centre.
Floral formula also shows cohesion and adhesion within parts of whorls and between whorls.
The floral diagram and floral formula in Figure 5.16 represents the mustard plant (Family: Brassicaceae).
5.9 Solanaceae
It is a large family, commonly called as the ‘potato family’.
It is widely distributed in tropics, subtropics and even temperate zones.
Vegetative Characters:
Plants mostly herbs, shrubs and rarely small trees
Stem: herbaceous rarely woody, aerial; erect, cylindrical, branched, solid or hollow, hairy or glabrous, underground stem in potato (Solanum tuberosum)
Leaves: alternate, simple, rarely pinnately compound, exstipulate; venation reticulate
Floral Characters
Inflorescence: Solitary, axillary or cymose as in Solanum
Flower: bisexual, actinomorphic
Calyx: sepals five, united, persistent, valvate aestivation
Corolla: petals five, united; valvate aestivation
Androecium: stamens five, epipetalous
Gynoecium: bicarpellary obligately placed, syncarpous; ovary superior, bilocular, placenta swollen with many ovules, axile
Fruits: berry or capsule
Seeds: many, endospermous
Floral Formula:
Economic Importance:
Many plants belonging to this family are source of food (tomato, brinjal, potato), spice (chilli); medicine (belladonna, ashwagandha); fumigatory (tobacco); ornamentals (petunia).
Summary
Flowering plants exhibit enormous variation in shape, size, structure, mode of nutrition, life span, habit and habitat.
They have well developed root and shoot systems.
Root system is either tap root or fibrous.
Generally, dicotyledonous plants have tap roots while monocotyledonous plants have fibrous roots.
The roots in some plants get modified for storage of food, mechanical support and respiration.
The shoot system is differentiated into stem, leaves, flowers and fruits.
The morphological features of stems like the presence of nodes and internodes, multicellular hair and positively phototropic nature help to differentiate the stems from roots.
Leaf is a lateral outgrowth of stem developed exogeneously at the node. These are green in colour to perform the function of photosynthesis.
Leaves exhibit marked variations in their shape, size, margin, apex and extent of incisions of leaf blade (lamina).
The flower is a modified shoot, meant for sexual reproduction.
The flowers are arranged in different types of inflorescences.
They exhibit enormous variation in structure, symmetry, position of ovary in relation to other parts, arrangement of petals, sepals, ovules etc.
After fertilisation, the ovary is modified into fruits and ovules into seeds.
Seeds either may be monocotyledonous or dicotyledonous.
They vary in shape, size and period of viability.
The floral characteristics form the basis of classification and identification of flowering plants.
This can be illustrated through semi- technical descriptions of families.
Hence, a flowering plant is described in a definite sequence by using scientific terms.
The floral features are represented in the summarised form as floral diagrams and floral formula.