biology

Morphology of Flowering Plants

A concise summary of the external structure of flowering plants, covering roots, stems, leaves, flowers, fruits, and seeds, along with their types, modifications, and arrangements, essential for quick revision.

Structural Organisation in Plants

The study of the diverse forms of life on Earth was initially based on observation of gross structural features, both external and internal. This descriptive biology, also known as natural history, provides a foundation for later reductionist biology, physiology, and evolutionary biology. The structural organisation of plants and animals, including the structural basis of physiological or behavioural phenomena, is described separately for convenience.

Morphology of Flowering Plants

This chapter describes the external structure or morphology of flowering plants (angiosperms), which show large diversity but are all characterised by the presence of roots, stems, leaves, flowers, and fruits. Understanding standard technical terms and definitions, as well as variations adapted to environment (e.g., for protection, climbing, storage), is essential for classification and understanding.

5.1 The Root

The root system is the underground part of a flowering plant.

  • Types of Roots:

    • Tap Root System: Develops from the direct elongation of the radicle, forming a primary root that grows inside the soil. It bears lateral roots (secondary, tertiary, etc.). Seen in most dicotyledonous plants, e.g., mustard plant.
    • Fibrous Root System: In monocotyledonous plants, the primary root is short-lived and replaced by a large number of roots that originate from the base of the stem. Seen in wheat plant.
    • Adventitious Roots: Roots arising from parts of the plant other than the radicle, e.g., grass, Monstera, and banyan tree.

  • Main Functions of the Root System:

    • Absorption of water and minerals from the soil.
    • Providing proper anchorage to the plant parts.
    • Storing reserve food material.
    • Synthesis of plant growth regulators.

  • Regions of the Root-tip:

    • Root Cap: A thimble-like structure covering the apex, protecting the tender tip as it makes its way through the soil.
    • Region of Meristematic Activity: Located a few millimetres above the root cap. Cells are very small, thin-walled, with dense protoplasm, and divide repeatedly.
    • Region of Elongation: Cells proximal to the meristematic region undergo rapid elongation and enlargement, responsible for root growth in length.
    • Region of Maturation: Proximal to the region of elongation, cells differentiate and mature. From this zone, some epidermal cells form fine, thread-like 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.

  • Distinguishing Features:

    • Bears nodes (where leaves are born) and internodes (portions between two nodes).
    • Bears buds, which may be terminal or axillary.
    • Generally green when young, later often becoming woody and dark brown.

  • Main Functions of the Stem:

    • Spreading out branches bearing leaves, flowers, and fruits.
    • Conducting water, minerals, and photosynthates.
    • Some stems perform functions of storage of food, support, protection, and 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, which can later develop 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.

  • Parts of a Typical Leaf:

    • Leaf Base: Attaches the leaf to the stem. May bear two small leaf-like structures called stipules. In monocotyledons, the leaf base expands into a sheath. In some leguminous plants, it may become swollen, called the pulvinus.
    • Petiole: Helps hold the blade to light. Long, thin, flexible petioles allow leaf blades to flutter in wind, cooling the leaf and bringing fresh air.
    • Lamina (Leaf Blade): The green expanded part of the leaf with veins and veinlets. The prominent middle vein is the midrib. Veins provide rigidity and act as channels for transport of water, minerals, and food materials.

  • 5.3.1 Venation: The arrangement of veins and veinlets in the lamina.

    • Reticulate Venation: Veinlets form a network. Characteristic of dicotyledonous plants.
    • Parallel Venation: Veins run parallel to each other within the lamina. Characteristic of most monocotyledons.

  • 5.3.2 Types of Leaves:

    • Simple Leaf: Lamina is entire, or if incised, the incisions do not touch the midrib. A bud is present in the axil of the petiole.
    • Compound Leaf: Incisions of the lamina reach up to the midrib, breaking it into a number of leaflets. A bud is present in the axil of the petiole, but not in the axil of the leaflets.
      • Pinnately Compound Leaf: A number of leaflets are present on a common axis called the rachis (which represents the midrib), e.g., neem.
      • Palmately Compound Leaf: The leaflets are attached at a common point, i.e., at the tip of the petiole, e.g., silk cotton.

  • 5.3.3 Phyllotaxy: The pattern of arrangement of leaves on the stem or branch. Usually of three types:

    • Alternate Phyllotaxy: A single leaf arises at each node in an alternate manner, e.g., china rose, mustard, and sunflower.
    • Opposite Phyllotaxy: A pair of leaves arise at each node and lie opposite to each other, e.g., Calotropis and guava plants.
    • Whorled Phyllotaxy: 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 where the shoot apical meristem changes to floral meristem. The arrangement of flowers on the floral axis is termed inflorescence.

  • Two Major Types of Inflorescences: Based on whether the apex develops into a flower or continues to grow.
    • Racemose Type: The main axis continues to grow, and flowers are borne laterally in an acropetal succession (older flowers at base, younger at apex).
    • Cymose Type: The main axis terminates in a flower, limiting its growth. Flowers are borne in a basipetal order (older flowers at apex, younger at base).

5.5 The Flower

The flower is the reproductive unit in angiosperms, meant for sexual reproduction. A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk (pedicel), called the thalamus or receptacle.

  • Floral Whorls:

    • Calyx (outermost) and Corolla are accessory organs.
    • Androecium and Gynoecium are reproductive organs.
    • In some flowers, like lily, the calyx and corolla are not distinct and are termed perianth.

  • Sexuality of Flowers:

    • Bisexual: Has both androecium and gynoecium.
    • Unisexual: Has either only stamens (male) or only carpels (female).

  • Symmetry of Flowers:

    • Actinomorphic (Radial Symmetry): Can be divided into two equal radial halves in any radial plane passing through the centre, e.g., mustard, datura, chilli.
    • Zygomorphic (Bilateral Symmetry): Can be divided into two similar halves only in one particular vertical plane, e.g., pea, gulmohur, bean, Cassia.
    • Asymmetric (Irregular): Cannot be divided into two similar halves by any vertical plane passing through the centre, e.g., canna.

  • Floral Appendage Multiplicity:

    • Trimerous: Floral appendages in multiples of 3.
    • Tetramerous: Floral appendages in multiples of 4.
    • Pentamerous: Floral appendages in multiples of 5.

  • Presence of Bracts:

    • Bracteate: Flowers with bracts (reduced leaf found at the base of the pedicel).
    • Ebracteate: Flowers without bracts.

  • Position of Floral Parts on Thalamus (Ovary Position):

    • Hypogynous Flower: Gynoecium occupies the highest position, while other parts are situated below it. The ovary is said to be superior, e.g., mustard, china rose, brinjal.
    • Perigynous Flower: Gynoecium is situated in the centre, and other parts are located on the rim of the thalamus almost at the same level. The ovary is said to be half inferior, e.g., plum, rose, peach.
    • Epigynous Flowers: The margin of the thalamus grows upward, enclosing the ovary completely and fusing with it. Other parts of the flower arise above the ovary. Hence, the ovary is said to be inferior, e.g., guava, cucumber, and ray florets of sunflower.

5.5.1 Parts of a Flower (Detailed)

Each flower typically has four floral whorls: calyx, corolla, androecium, and gynoecium.

  • 5.5.1.1 Calyx:

    • The outermost whorl, members called sepals.
    • Generally green, leaf-like, and protect the flower in the bud stage.
    • May be gamosepalous (sepals united) or polysepalous (sepals free).

  • 5.5.1.2 Corolla:

    • Composed of petals.
    • Usually brightly coloured to attract insects for pollination.
    • May be gamopetalous (petals united) or polypetalous (petals free).
    • Shape and colour vary greatly; can be tubular, bell-shaped, funnel-shaped, or wheel-shaped.

  • Aestivation: The mode of arrangement of sepals or petals in the floral bud with respect to 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: 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, the largest petal (standard) overlaps two lateral petals (wings), which in turn overlap two smallest anterior petals (keel).

  • 5.5.1.3 Androecium:

    • Composed of stamens, which represent the male reproductive organ.
    • Each stamen consists of a stalk or a filament and an anther.
    • Each anther is usually bilobed, with two chambers called pollen-sacs, where pollen grains are produced.
    • A sterile stamen is called a staminode.
    • Attachment to other members:
      • Epipetalous: Stamens attached to the petals, e.g., brinjal.
      • Epiphyllous: Stamens attached to the perianth, e.g., lily.
    • Union among themselves (Cohesion):
      • Polyandrous: Stamens remain free.
      • Monoadelphous: Stamens united into one bunch/bundle, e.g., china rose.
      • Diadelphous: Stamens united into two bundles, e.g., pea.
      • Polyadelphous: Stamens united into more than two bundles, e.g., citrus.
    • Variation in filament length can occur within a flower, e.g., Salvia and mustard.

  • 5.5.1.4 Gynoecium:

    • The female reproductive part of the flower, made up of one or more carpels.
    • Each carpel consists of three parts:
      • Ovary: The enlarged basal part. Bears one or more ovules attached to a flattened, cushion-like placenta.
      • Style: The elongated tube that connects the ovary to the stigma.
      • Stigma: Usually at the tip of the style, the receptive surface for pollen grains.
    • Carpel Arrangement (when more than one carpel is present):
      • Apocarpous: Carpels are free, e.g., lotus and rose.
      • Syncarpous: Carpels are fused, e.g., mustard and tomato.
    • After fertilisation, 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 ovules are borne on this ridge forming two rows, e.g., pea.
    • Axile Placentation: The placenta is axial, and ovules are attached to it in a multilocular ovary, e.g., china rose, tomato, and lemon.
    • Parietal Placentation: Ovules develop on the inner wall of the ovary or on the peripheral part. The ovary is typically one-chambered but may become two-chambered due to a false septum, e.g., mustard and Argemone.
    • Free Central Placentation: Ovules are borne on a central axis, and septa are absent, e.g., Dianthus and Primrose.
    • Basal Placentation: The placenta develops at the base of the ovary, and a single ovule is attached to it, e.g., sunflower, marigold.

5.6 The Fruit

The fruit is a characteristic feature of flowering plants. It is a mature or ripened ovary, developed after fertilisation.

  • Parthenocarpic Fruit: A fruit formed without fertilisation of the ovary.
  • Structure: Generally consists of a pericarp (wall) and seeds.
    • Pericarp: May be dry or fleshy. When thick and fleshy, it differentiates into:
      • Epicarp: The outer layer.
      • Mesocarp: The middle layer.
      • Endocarp: The inner layer.
  • Drupe: A type of fruit seen in mango and coconut. They develop from monocarpellary superior ovaries and are one-seeded. In mango, the mesocarp is fleshy and edible, and the endocarp is stony and hard. In coconut, the mesocarp is fibrous.

5.7 The Seed

Seeds develop from ovules after fertilisation.

  • Structure: A seed is made up of a seed coat and an embryo. The embryo consists of a radicle, an embryonal axis, and one or two cotyledons.

  • 5.7.1 Structure of a Dicotyledonous Seed:

    • Seed Coat: The outermost covering, with two layers: the outer testa and the inner tegmen.
    • Hilum: A scar on the seed coat where the developing seeds were attached to the fruit.
    • Micropyle: A small pore located above the hilum.
    • Embryo: Consists of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials. The radicle and plumule are present at the two ends of the embryonal axis.
    • Endospermic Seeds: Have endosperm (food storing tissue formed from double fertilisation), e.g., castor.
    • Non-endospermous Seeds: Endosperm is not present in mature seeds, e.g., bean, gram, and pea.

  • 5.7.2 Structure of a Monocotyledonous Seed:

    • Generally endospermic, e.g., maize, but some are non-endospermic, e.g., orchids.
    • In cereals like maize, the seed coat is membranous and generally fused with the fruit wall.
    • The endosperm is bulky and stores food.
    • Aleurone Layer: A proteinous layer that separates the outer covering of the endosperm from the embryo.
    • Embryo: Small and situated in a groove at one end of the endosperm. It consists of:
      • One large, shield-shaped cotyledon known as scutellum.
      • A short axis with a plumule and a radicle.
      • The plumule and radicle are enclosed in sheaths called coleoptile and coleorhiza, respectively.

5.8 Semi-technical Description of a Typical Flowering Plant

The description of a flowering plant is brief, in simple and scientific language, and presented in a proper sequence.

  • Sequence of Description:

    1. Habit.
    2. Vegetative characters: roots, stem, and leaves.
    3. Floral characters: inflorescence and flower parts.
    4. Finally, a floral diagram and a floral formula are presented.

  • Floral Formula Symbols:

    • Br: Bracteate
    • K: Calyx
    • C: Corolla
    • P: Perianth
    • A: Androecium
    • G: Gynoecium
    • : Superior ovary
    • : Inferior ovary
    • ♂: Male flower
    • ♀: Female flower
    • ⚥: Bisexual flower
    • ⊕: Actinomorphic (radial symmetry) flower
    • %: Zygomorphic (bilateral symmetry) flower
    • Fusion is indicated by enclosing the figure within brackets, e.g., K(5).
    • Adhesion (fusion between different whorls) is indicated by a line drawn above the symbols of the floral parts, e.g., A C (epipetalous).

  • Floral Diagram:

    • Provides information about the number of parts of a flower, their arrangement, and their relation to one another.
    • A dot on the top of the floral diagram represents the position of the mother axis with respect to the flower.
    • Calyx, corolla, androecium, and gynoecium are drawn in successive whorls, with the calyx being the outermost and the gynoecium in the centre.
    • Shows cohesion and adhesion within parts of whorls and between whorls.
    • Example: Floral formula ⊕ ⚥ K C A G represents the mustard plant (Family: Brassicaceae).

5.9 Solanaceae (The Potato Family)

A large family widely distributed in tropics, subtropics, and 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 found in potato (Solanum tuberosum).
    • Leaves: Alternate, simple (rarely pinnately compound), exstipulate; venation is reticulate.

  • Floral Characters:

    • Inflorescence: Solitary, axillary, or cymose (as in Solanum).
    • Flower: Bisexual, actinomorphic.
    • Calyx: Five sepals, united, persistent, with valvate aestivation.
    • Corolla: Five petals, united, with valvate aestivation.
    • Androecium: Five stamens, epipetalous (attached to petals).
    • Gynoecium: Bicarpellary (two carpels), obligately placed, syncarpous (fused carpels). Ovary is superior, bilocular (two chambers), placenta swollen with many ovules, with axile placentation.
    • Fruits: Berry or capsule.
    • Seeds: Many, endospermous.
    • Floral Formula: ⊕ ⚥ K C A G.

  • Economic Importance:

    • Food: Tomato, brinjal, potato.
    • Spice: Chilli.
    • Medicine: Belladonna, ashwagandha.
    • Fumigatory: Tobacco.
    • Ornamentals: Petunia.