Morphology in Flowering Plants Complete Notes

Historical Context of Plant Biology

• Early descriptions of life forms relied on observation using naked eyes and magnifying lenses.
• Descriptions focused on gross structural features, both external and internal.
• Observable living phenomena were also recorded.
• Before experimental biology (physiology) became established, biology was primarily natural history.
• Detailed descriptions of life forms later became valuable in reductionist biology.
• Reductionist biology shifted focus to living processes rather than just describing life forms.
• Detailed descriptions helped frame research questions in physiology and evolutionary biology.
• The unit will cover the structural organization of plants and animals, including the structural basis of physiological and behavioral phenomena.
• Morphological and anatomical features will be presented separately for plants and animals.

Katherine Esau (1898 – 1997)

• Born in Ukraine in 1898.
• Studied agriculture in Russia and Germany.
• Received her doctorate in 1931 in the United States.
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• Early publications reported that the curly top virus spreads through plants via the phloem tissue.
• Dr. Esau’s Plant Anatomy, published in 1954, took a dynamic, developmental approach to enhance the understanding of plant structure.
• The book had a worldwide impact and revived the discipline of plant anatomy.
• The Anatomy of Seed Plants by Katherine Esau was published in 1960 and was 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, director of Anatomy and Morphology, Missouri Botanical Garden, noted that she ‘absolutely dominated’ the field of plant biology even at the age of 99.

Chapter 5: Morphology of Flowering Plants

• Angiosperms exhibit a large 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 (e.g., habitats, protection, climbing, storage).
• All weeds have roots, stems, and leaves; they may also bear flowers and fruits.
• The underground part of the flowering plant is the root system, while the portion above the ground is the shoot system.

5.1 The Root

• In most dicotyledonous plants, the direct elongation of the radicle leads to the formation of the primary root.
• The primary root grows inside the soil.
• It bears lateral roots of several orders (secondary, tertiary, etc.).
• The primary root and its branches constitute the tap root system, as seen in the mustard plant.
• In monocotyledonous plants, the primary root is short-lived and 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 (grass, Monstera, banyan tree), roots arise from parts of the plant other than the radicle and are called adventitious roots.
• Main functions of the root system:
  • Absorption of water and minerals from the soil.
  • Providing 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.
• The root cap 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.
  • Cells in this region are small, thin-walled, and have dense protoplasm.
  • These cells divide repeatedly.
• Cells proximal to the meristematic region undergo rapid elongation and enlargement.
  • This region is responsible for the growth of the root in length and is called the region of elongation.
• Cells of the elongation zone gradually differentiate and mature.
  • The zone proximal to the region of elongation is called the region of maturation.
• Epidermal cells in the maturation region form very fine and delicate, thread-like structures called root hairs.
• 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.
  • Nodes are the regions where leaves are born.
  • 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.
• Main functions of the stem:
  • Spreading out branches bearing leaves, flowers, and fruits.
  • Conducting water, minerals, and photosynthates.
  • Storage of food.
  • Support.
  • Protection.
  • 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 light.
• Long, thin, flexible petioles allow leaf blades to flutter in 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 the lamina varies in different leaves.

5.3.1 Venation

• The arrangement of veins and the veinlets in the lamina of the leaf is termed venation.
• When the veinlets form a network, the venation is termed reticulate.
• When the veins run parallel to each other within a lamina, the venation is termed parallel.
• Leaves of dicotyledonous plants generally possess reticulate venation.
• 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 the 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:
  • Pinnately compound leaf: leaflets are present on a common axis, the rachis, which represents the midrib of the leaf (e.g., neem).
  • Palmately compound leaves: leaflets are attached at a common point (i.e., at the tip of the petiole (e.g., silk cotton).

5.3.3 Phyllotaxy

• Phyllotaxy is the pattern of arrangement of leaves on the stem or branch.
• There are usually three types: alternate, opposite, and whorled.
• Alternate type: A single leaf arises at each node in an alternate manner (e.g., china rose, mustard, and sunflower plants).
• Opposite type: A pair of leaves arise at each node and lie opposite to each other (e.g., Calotropis and guava plants).
• Whorled type: More than two leaves arise at a node and form a whorl (e.g., 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 inflorescence.
• Two major types of inflorescences:
  • Racemose: The main axis continues to grow, and the flowers are borne laterally in an acropetal succession.
  • Cymose: 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 and 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 the 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 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).
• Actinomorphic: A flower can be divided into two equal radial halves in any radial plane passing through the center (e.g., mustard, datura, chilli).
• Zygomorphic: A flower can be divided into two similar halves only in one particular vertical plane (e.g., pea, gulmohur, bean, Cassia).
• Asymmetric (irregular): A flower 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 are ebracteate.
• Based on the position of the calyx, corolla, and androecium in respect of the ovary on the thalamus:
  • Hypogynous: The gynoecium occupies the highest position while the other parts are situated below it; the ovary is superior (e.g., mustard, china rose, brinjal).
  • Perigynous: The gynoecium is situated in the center, and other parts of the flower are located on the rim of the thalamus almost at the same level; the ovary is half inferior (e.g., plum, rose, peach).
  • Epigynous: The margin of the thalamus grows upward enclosing the ovary completely and getting fused with it; the other parts of the flower arise above the ovary; the ovary is inferior (e.g., 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: 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.
• Sepals are generally 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

• The corolla is composed of petals.
• Petals are usually brightly colored to attract insects for pollination.
• Like the calyx, the corolla may also be gamopetalous (petals united) or polypetalous (petals free).
• The shape and color of the corolla vary greatly in plants.
• The corolla may be tubular, bell-shaped, funnel-shaped, or wheel-shaped.
• Aestivation: The mode of arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl.
  • Valvate: Sepals or petals in a whorl just touch one another at the margin, without overlapping (e.g., Calotropis).
  • Twisted: One margin of the appendage overlaps that of the next one and so on (e.g., china rose, lady’s finger, and cotton).
  • Imbricate: The margins of sepals or petals overlap one another but not in any particular direction (e.g., Cassia and gulmohur).
  • Vexillary (papilionaceous): 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).

5.5.1.3 Androecium

• The 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 a flower may be united with other members such as petals or among themselves.
  • Epipetalous: When stamens are attached to the petals (e.g., brinjal).
  • Epiphyllous: When stamens are attached to the perianth (e.g., flowers of lily).
• The stamens in a flower may either remain free (polyandrous) or may be united in varying degrees.
  • Monoadelphous: The stamens may be united into one bunch or one bundle (e.g., china rose).
  • Diadelphous: The stamens are united into two bundles (e.g., pea).
  • Polyadelphous: The stamens are united into more than two bundles (e.g., citrus).
• There may be a variation in the length of filaments within a flower, as in Salvia and mustard.

5.5.1.4 Gynoecium

• The gynoecium is the female reproductive part of the flower and is made up of one or more carpels.
• A carpel consists of three parts: stigma, style, and ovary.
• The 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.
  • 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.
  • Axile Placentation: The placenta is axial, and the ovules are attached to it in a multilocular ovary, as in china rose, tomato, and lemon.
  • Parietal Placentation: The ovules develop on the inner wall of the ovary or on the peripheral part; The ovary is one-chambered but it becomes two-chambered due to the formation of the false septum, e.g., mustard and Argemone.
  • Free Central Placentation: The ovules are borne on a central axis, and septa are absent, as in Dianthus and Primrose.
  • Basal Placentation: The placenta develops at the base of the ovary, and a single ovule is attached to it, as in sunflower and 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 the 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.
• Drupes 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 such seeds are 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 the endosperm separates the embryo by a proteinous layer called the 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 the 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 the plant, a floral diagram and a floral formula are presented.
• The floral formula is represented by some symbols:
  • Br stands for bracteate.
  • K stands for calyx.
  • C stands for corolla.
  • P stands for perianth.
  • A stands for androecium.
  • G stands for gynoecium.
  • $G$ stands for superior ovary.
  • $\overline{G}$ stands for inferior ovary.
  • \male stands for male.
  • \female stands for female.
  • \bisexual stands for bisexual plants.
  • $\oplus$ stands for actinomorphic.
  • stands for zygomorphic nature of flower.
• Fusion is indicated by enclosing the figure within a 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 center.
• The floral formula also shows cohesion and adhesion within parts of whorls and between whorls.
• The floral diagram and floral formula represent the mustard plant (Family: Brassicaceae).

5.9 Solanaceae

• It is a large family, commonly called the ‘potato family’.
• It is widely distributed in tropics, subtropics, and even temperate zones.
• Vegetative Characters:
  • Plants are 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: \oplus \female K{(5)} C{(5)} A{5} \overline{G}{(2)}
• Economic Importance:
  • Many plants belonging to this family are a source of food (tomato, brinjal, potato), spice (chili), medicine (belladonna, ashwagandha), fumigatory (tobacco), and ornamentals (petunia).