Chapter 2.1: Morphology of Flowering Plants

1. The Root

• Definition: The root is the non-green, underground, positively geotropic (grows towards gravity), and negatively phototropic (grows away from light) part of the plant, which develops from the radicle of the embryo.

• Regions of the Root:

  • Root Cap: A thimble-like structure that covers the root apex, protecting the tender apical meristem as it makes its way through the soil.
  • Region of Meristematic Activity: A few millimeters above the root cap, this region has actively dividing cells which are small, thin-walled, and with dense protoplasm.
  • Region of Elongation: The cells proximal to the meristematic region undergo rapid elongation and enlargement and are responsible for the growth of the root in length.
  • Region of Maturation: The cells from the elongation zone differentiate and mature. From this region, very fine and delicate, thread-like structures called root hairs arise, which absorb water and minerals from the soil.

• Types of Root Systems:

  • Tap Root System: This system consists of a main, primary root that grows vertically down into the soil, and gives rise to lateral branches called secondary and tertiary roots. It is characteristic of dicotyledonous plants. e.g., Mustard, Gram, Carrot, Radish.
  • Fibrous Root System: In this system, the primary root is short-lived and is replaced by a large number of thin, moderately branching roots originating from the base of the stem. It is characteristic of monocotyledonous plants. e.g., Wheat, Rice, Grass.
  • Adventitious Root System: These are roots that arise from any part of the plant other than the radicle or its branches. They can grow from the stem, leaves, or even branches. e.g., Grass, Monstera, Banyan tree.

• Modifications of Root:

  • For Storage:
    • Tap Roots:
      • Conical: The root is cone-shaped, broad at the base and gradually tapering towards the apex. e.g., Carrot.
      • Fusiform: The root is swollen in the middle and tapers towards both ends. e.g., Radish.
      • Napiform: The root is spherical at the base and tapers sharply towards the apex. e.g., Turnip.
    • Adventitious Roots:
      • Tuberous: These roots are swollen and have no definite shape. They are produced singly and not in clusters. e.g., Sweet Potato, Mirabilis.
  • For Support:
    • Prop Roots: These roots arise from the horizontal branches of the tree, grow vertically downwards, and penetrate the soil, providing support to the heavy branches. e.g., Banyan tree.
    • Stilt Roots: These are thick, short, and obliquely growing roots that develop from the lower nodes of the stem to provide extra support to the plant. e.g., Maize, Sugarcane.
  • For Respiration:
    • Pneumatophores: In plants growing in saline swamps (mangroves), some of the roots grow vertically upwards from the soil (negatively geotropic) and have numerous pores (pneumathodes) through which air diffuses. e.g., Rhizophora.
  Breathing Roots In marshy areas, the soil is waterlogged and lacks oxygen. Pneumatophores are an amazing adaptation that allows trees like Rhizophora to "breathe" atmospheric oxygen through specialized pores.

  2. The Stem

• Definition: The stem is the ascending part of the plant axis that develops from the plumule of the embryo. It bears branches, leaves, flowers, and fruits.

• Features:

  • Nodes: The regions of the stem where leaves are borne.
  • Internodes: The portions of the stem between two nodes.
  • Buds: Young, immature, and condensed shoots. They can be terminal (at the apex of the stem) or axillary (in the axils of leaves).

• Modifications of Stem:

  • Underground Modifications (For Storage and Perennation):
    • Tuber: The swollen, fleshy, underground tip of a stem branch which has stored food and bears buds (eyes) for vegetative propagation. e.g., Potato.
    • Rhizome: A prostrate, dorsiventrally flattened, underground stem that grows horizontally. It has nodes, internodes, and scale leaves. e.g., Ginger, Turmeric.
    • Corm: A condensed, fleshy, vertically growing, underground stem with a flattened base. It bears buds in the axils of scale leaves. e.g., Colocasia, Gladiolus.
  • Sub-aerial Modifications (For Vegetative Propagation):
    • Runner: A slender, prostrate branch with long internodes that creeps on the ground and roots at the nodes. e.g., Grass, Oxalis.
    • Stolon: A slender lateral branch that arises from the base of the main axis and after growing aerially for some time, arches downwards to touch the ground. e.g., Strawberry, Mint.
    • Sucker: A lateral branch that originates from the basal and underground portion of the main stem, grows horizontally beneath the soil, and then comes out obliquely upwards giving rise to a leafy shoot. e.g., Banana, Pineapple, Chrysanthemum.
    • Offset: A lateral branch with short internodes and each node bearing a rosette of leaves and a tuft of roots. It is found in aquatic plants. e.g., Pistia, Eichhornia.
  • Aerial Modifications:
    • Stem Tendrils: Slender, spirally coiled structures that develop from axillary buds and help the plant to climb. e.g., Gourds (cucumber, pumpkins), Grapevines.
    • Thorns: Woody, straight, and pointed structures that develop from axillary buds. They protect the plant from browsing animals. e.g., Citrus, Bougainvillea.
    • Phylloclade: In some arid plants, the stem is modified into a flattened (e.g., Opuntia) or fleshy cylindrical (e.g., Euphorbia) green structure that carries out photosynthesis.
    • Cladode: A green, flattened or cylindrical stem of limited growth (usually one internode long) which has taken up the function of photosynthesis. e.g., Asparagus.

3. The Leaf

• Definition: A lateral, generally flattened, green, and expanded structure borne on the stem at the node, primarily responsible for photosynthesis.
• Parts of a Simple Leaf:
  • Leaf Base: The part of the leaf that is attached to the stem.
  • Petiole: The stalk that connects the leaf blade to the stem.
  • Lamina (Leaf Blade): The broad, green, and expanded part of the leaf with veins and veinlets.
• Venation: The arrangement of veins and veinlets in the lamina of the leaf.
  • Reticulate Venation: The veinlets form a network. Characteristic of dicots.
  • Parallel Venation: The veins run parallel to each other within the lamina. Characteristic of monocots.
• Types of Leaves:
  • Simple Leaf: The lamina is entire or, when incised, the incisions do not touch the midrib. e.g., Mango, Guava.
  • Compound Leaf: The incisions of the lamina reach up to the midrib, breaking it into a number of smaller parts called leaflets.
    • Pinnately Compound: The leaflets are present on a common axis called the rachis, which represents the midrib of the leaf. e.g., Neem.
    • Palmately Compound: The leaflets are attached at a common point, i.e., at the tip of the petiole. e.g., Silk Cotton.
• Phyllotaxy: The pattern of arrangement of leaves on the stem or a branch.
  • Alternate: A single leaf arises at each node in an alternate manner. e.g., China rose, Mustard.
  • Opposite: A pair of leaves arise at each node and lie opposite to each other. e.g., Calotropis, Guava.
  • Whorled: More than two leaves arise at a node and form a whorl. e.g., Alstonia.
• Modifications of Leaves:
  • For Mechanical Support (Tendrils): The entire leaf or a part of it is modified into a slender, wiry, and coiled structure for climbing. e.g., Peas.
  • For Protection (Spines): The leaves are modified into hard, stiff, and sharp-pointed structures to reduce water loss and protect against grazing. e.g., Cacti.
  • For Storage (Bulb): The leaves become fleshy and store food. e.g., Onion, Garlic.
  • For Reproduction: In some plants, adventitious buds develop on the leaves which can give rise to new plants. e.g., Bryophyllum.
  • Insectivorous Plants:
    • Pitcher Plant: The lamina is modified into a pitcher-like structure to trap and digest insects.
    • Venus-fly-trap: The leaf is modified into a trap with two lobes that can snap shut to catch insects.

4. Inflorescence

• Definition: The arrangement of flowers on the floral axis (peduncle).
• Types:
  • Racemose: The main axis continues to grow indefinitely, and the flowers are borne laterally in an acropetal succession (older flowers are at the base and younger ones are at the apex). e.g., Radish, Mustard.
  • Cymose: The main axis terminates in a flower, hence the growth is limited. The flowers are borne in a basipetal order (older flowers are at the apex and younger ones are at the base). e.g., Jasmine, Calotropis.

Differences between Racemose and Cymose Inflorescence

5. The Flower

• Definition: The reproductive unit in angiosperms, meant for sexual reproduction.

• Structure of a Typical Flower: A typical flower has four distinct whorls of floral appendages attached to the swollen end of the pedicel, called the thalamus or receptacle. These are Calyx, Corolla, Androecium, and Gynoecium.

• Bracteate/Ebracteate:

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

• Symmetry:

  • Actinomorphic (Radial Symmetry): The flower can be divided into two equal radial halves in any radial plane passing through the center. e.g., Mustard, Datura, Chilli.
  • Zygomorphic (Bilateral Symmetry): The flower can be divided into two similar halves only in one particular vertical plane. e.g., Pea, Gulmohar, Bean, Cassia.

• Floral Appendages:

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

• Completeness:

  • Complete: A flower with all four floral whorls.
  • Incomplete: A flower lacking one or more floral whorls.

• Non-Essential Whorls (Accessory Whorls):

  • Calyx: The outermost whorl composed of sepals. They are usually green, leaf-like, and protect the flower in the bud stage.
    • Gamosepalous: Sepals are united.
    • Polysepalous: Sepals are free.
  • Corolla: The whorl inner to the calyx, composed of petals. They are usually brightly colored to attract insects for pollination.
    • Gamopetalous: Petals are united.
    • Polypetalous: Petals are free.
  • Perianth: In some flowers, the calyx and corolla are not distinct and are collectively called the perianth. The individual units are called tepals. e.g., Lily.
  • 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.
    • Imbricate: The margins of sepals or petals overlap one another but not in any particular direction. e.g., Cassia, Gulmohar.
    • Vexillary (Papilionaceous): 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.

• Essential Whorls (Reproductive Whorls):

  • Androecium (Stamen): The male reproductive part, composed of stamens. Each stamen consists of a stalk or a filament and an anther.
    • Cohesion of Stamens:
      • Monadelphous: Stamens are united into one bundle or bunch. e.g., China rose.
      • Diadelphous: Stamens are united into two bundles. e.g., Pea.
      • Polyadelphous: Stamens are united into more than two bundles. e.g., Citrus.
      • Syngenesious: Anthers are united, but filaments are free. e.g., Sunflower.
      • Synandrous: Stamens are united throughout their whole length (both filaments and anthers). e.g., Cucurbita.
    • Adhesion of Stamens:
      • Epipetalous: Stamens are attached to the petals. e.g., Brinjal.
      • Epiphyllous: Stamens are attached to the perianth. e.g., Lily.
    • Number of Lobes in Anther:
      • Monothecous: Anther with a single lobe.
      • Dithecous: Anther with two lobes.
  • Gynoecium (Pistil/Carpel): The female reproductive part of the flower, composed of one or more carpels. A carpel consists of three parts: stigma, style, and ovary.
    • Position of Ovary:
      • Hypogynous: The gynoecium occupies the highest position while the other parts are situated below it. The ovary is said to be superior. e.g., Mustard, China rose.
      • 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 said to be half inferior. e.g., Plum, Rose.
      • 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 said to be inferior. e.g., Guava, Cucumber.
    • Cohesion of Carpels:
      • Apocarpous: More than one carpel is present, and they are free. e.g., Lotus, Rose.
      • Syncarpous: More than one carpel is present, and they are fused. e.g., Mustard, Tomato.
    • Number of Locules in Ovary:
      • Unilocular: Ovary with one chamber.
      • Bilocular: Ovary with two chambers.
      • Multilocular: Ovary with more than two chambers.
    • Placentation: The arrangement of ovules within the ovary.
      • Marginal: 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.
      • Axile: The placenta is axial and the ovules are attached to it in a multilocular ovary. e.g., China rose, Tomato, Lemon.
      • Parietal: The ovules develop on the inner wall of the ovary or on the peripheral part. e.g., Mustard, Argemone.
      • Free Central: The ovules are borne on a central axis and the septa are absent. e.g., Dianthus, Primrose.
      • Basal: The placenta develops at the base of the ovary and a single ovule is attached to it. e.g., Sunflower, Marigold.

6. The Floral Formula

A floral formula is a concise way to represent the structure of a flower using specific letters, numbers, and symbols. It provides a summary of the floral characteristics.

Symbols Used in Floral Formula:

How to Write a Floral Formula (with Example):

Let's construct the floral formula for a Mustard flower (Brassica):

1. Bracts: The flower is ebracteate. So, we write Ebr.
2. Symmetry: It is actinomorphic. So, we add ⊕.
3. Sexuality: It is bisexual. So, we add ⚥.
4. Calyx (K): There are 4 sepals, arranged in two whorls of two. They are free (polysepalous). So, we write K₂₊₂.
5. Corolla (C): There are 4 petals, and they are free (polypetalous). So, we write C₄.
6. Androecium (A): There are 6 stamens, arranged in two whorls. The outer whorl has 2 short stamens, and the inner whorl has 4 long stamens. They are free. So, we write A₂₊₄.
7. Gynoecium (G): There are 2 carpels, and they are fused (syncarpous). The ovary is superior. So, we write G₍₂₎ (the line underneath indicates a superior ovary).

Combining these gives the complete floral formula for Mustard:

Ebr ⊕ ⚥ K₂₊₂ C₄ A₂₊₄ G₍₂₎