Comprehensive Biology Notes: Morphology of Flowering Plants

The Root System

  • Definition: The root system consists of the underground portions of the plant, primarily the primary root and its various branches.

  • Types of Roots:

    • Tap Root System: Found in most dicotyledonous plants. It develops from the radicle. The primary root persists and produces lateral roots (secondary, tertiary, etc.). Example: Mustard plant.

    • Fibrous Root System: Characteristic of monocotyledonous plants. The primary root is short-lived and is replaced by a large number of roots that originate from the base of the stem. Example: Wheat.

    • Adventitious Roots: Roots that originate from parts of the plant other than the radicle. Examples: Grass, Monstera, and the Banyan tree.

  • Regions of the Root:

    • Root Cap: A thimble-like structure covering the root tip. It protects the tender meristematic cells and helps the root make its way through the soil.

    • Region of Meristematic Activity: Located a few millimeters above the root cap. Cells here are very small, thin-walled, and possess dense protoplasm. They divide continuously and have abundant plasmodesmatal connections.

    • Region of Elongation: Proximally located to the meristematic zone. Cells undergo rapid elongation and enlargement, leading to the growth of the root in length. Vacuolation is prominent here.

    • Region of Maturation: Located proximal to the elongation zone. Cells differentiate and mature.

    • Root Hairs: Formed in the region of maturation. These are very fine, delicate, thread-like structures that increase the surface area for the absorption of water and minerals from the soil.

  • Modifications of Root:

    • For Food Storage:

      • Tap roots: Carrot, Turnip.

      • Adventitious roots: Sweet Potato, Asparagus.

    • For Support:

      • Prop Roots: Pillar-like roots hanging from the Banyan tree.

      • Stilt Roots: Roots arising from the lowest nodes of the stem for support. Examples: Maize, Sugarcane.

    • For Respiration:

      • Pneumatophores: Roots that grow vertically upwards from the soil to get oxygen for respiration, typically found in swampy or marshy areas where oxygen is low. Example: Rhizophora.

  • Primary Functions of Roots:

    • Absorption of water and minerals from the soil.

    • Providing proper anchorage to the plant parts.

    • Storing reserve food material.

    • Synthesis of plant growth regulators (PGRs).

The Stem

  • General Characteristics: Spreads out branches bearing leaves, flowers, and fruits; conducts water, minerals, and photosynthates; provides support.

  • Modifications of Stem:

    • Underground Stems for Food Storage: These also act as organs of perennation to tide over conditions unfavorable for growth. Examples: Potato, Ginger, Turmeric, Zaminkand, and Colocasia.

    • Stem Tendrils: Develop from axillary buds and help plants to climb. Examples: Gourds (Cucumber, Pumpkins, Watermelon) and Grapevines.

    • Thorns: Modified axillary buds that are woody, straight, and pointed. They protect the plant from browsing animals. Examples: Citrus, Bougainvillea.

    • Photosynthetic Stems (Phylloclades): Modified to perform photosynthesis in arid regions.

      • Flat stems: Opuntia.

      • Fleshy cylindrical stems: Euphorbia.

  • Vegetative Propagation Modifications:

    • Runners: Underground stems of some plants spread to new niches and when older parts die, new plants are formed. Examples: Grass, Strawberry.

    • Stolons: A slender lateral branch arises from the base of the main axis and, after growing aerially for some time, arches downwards to touch the ground. Examples: Mint, Jasmine.

    • Offsets: A lateral branch with short internodes where each node bears a rosette of leaves and a tuft of roots; typically found in aquatic plants. Examples: Pistia, Eichhornia (Water Hyacinth).

    • Suckers: Lateral branches originate from the basal and underground portion of the main stem, grow horizontally beneath the soil, and then come out obliquely upward giving rise to leafy shoots. Examples: Banana, Pineapple, Chrysanthemum.

The Leaf

  • Parts of a Leaf:

    • Leaf Base: Connects the petiole to the stem and may bear two small leaf-like structures called stipules.

      • In monocots, the leaf base expands into a sheath covering the stem partially or wholly.

      • In some leguminous plants, the leaf base may become swollen, called the pulvinus.

    • Petiole: A long, thin, flexible structure that allows leaf blades to flutter in the wind, cooling the leaf and bringing fresh air to the surface. It helps orient the leaf toward light.

    • Lamina (Leaf Blade): The green, expanded part of the leaf with veins and veinlets. The central prominent vein is the midrib. Veins provide rigidity to the blade and act as channels for transport.

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

    • Reticulate: Veinlets form a network. Typical of Dicots.

    • Parallel: Veins run parallel to each other. Typical of Monocots.

  • Types of Leaves:

    • Simple Leaf: Lamina is entire or, if incised, the incisions do not reach the midrib.

    • Compound Leaf: Incisions reach the midrib, breaking the lamina into several leaflets.

      • Pinnately Compound: Leaflets are present on a common axis called the rachis (represents the midrib). Example: Neem.

      • Palmately Compound: Leaflets are attached at a common point at the tip of the petiole. Example: Silk Cotton.

  • Phyllotaxy: The pattern of arrangement of leaves on the stem or branch.

    • Alternate: A single leaf arises at each node in an alternate manner. Examples: China rose, Mustard, Sunflower.

    • Opposite: A pair of leaves arise at each node opposite to each other. Examples: Calotropis, Guava.

    • Whorled: More than two leaves arise at each node. Example: Alstonia.

  • Modifications of Leaves:

    • Tendrils: For climbing. Example: Pea.

    • Spines: For defense. Example: Cacti.

    • Storage: Fleshy leaves of Onion and Garlic.

    • Photosynthetic Petiole: In Australian Acacia, leaves are small and short-lived, so the petioles expand and become green to synthesize food.

    • Insect Traps: Pitcher plant and Venus flytrap.

The Inflorescence and Flower

  • Inflorescence: The arrangement of flowers on the floral axis. The flower is a modified shoot where the shoot apical meristem changes to a floral meristem.

    • Racemose: Main axis continues to grow; flowers are borne laterally in an acropetal succession (oldest at the base).

    • Cymose: Main axis terminates in a flower, limiting growth; flowers are borne in basipetal succession (youngest at the base).

  • Flower Structures:

    • Reproductive Unit: The reproductive organ in angiosperms.

    • Whorls:

      • Calyx: Outermost whorl, composed of sepals (usually green and leaf-like).

      • Corolla: Composed of petals (usually brightly colored to attract insects).

      • Perianth: When calyx and corolla are not distinct. Example: Lily.

      • Androecium: Composed of stamens (male reproductive organ).

      • Gynoecium: Composed of one or more carpels (female reproductive organ).

  • Flower Symmetry:

    • Actinomorphic (Radial): Can be divided into two equal halves in any radial plane passing through the center. Examples: Mustard, Datura, Chilli.

    • Zygomorphic (Bilateral): Can be divided into two similar halves only in one particular vertical plane. Examples: Pea, Gulmohur, Bean, Cassia.

    • Asymmetric (Irregular): Cannot be divided into two similar halves by any vertical plane. Example: Canna.

  • Flower Types (Position of Ovary):

    • Hypogynous: Gynoecium occupies the highest position; ovary is superior (GsuperiorG_{\text{superior}}). Examples: Mustard, China rose, Brinjal.

    • Perigynous: Gynoecium is situated in the center and other parts are on the rim of the thalamus at the same level; ovary is half-inferior (G−G_{-}). Examples: Plum, Rose, Peach.

    • Epigynous: Thalamus margin grows upward enclosing the ovary completely; other parts arise above the ovary; ovary is inferior (Gˉ\bar{G}). Examples: Guava, Cucumber, and ray florets of sunflower.

Floral Parts Detail

  • Calyx and Corolla:

    • Gamosepalous/Gamopetalous: Sepals or petals are united.

    • Polysepalous/Polypetalous: Sepals or petals are free.

  • Aestivation: Mode of arrangement of sepals or petals in the floral bud.

    • Valvate: Margins touch without overlapping. Example: Calotropis.

    • Twisted: One margin overlaps the next. Examples: China rose, Lady's finger, Cotton.

    • Imbricate: Margins overlap but not in a particular direction. Examples: Gulmohur, Cassia.

    • Vexillary (Papilionaceous): The largest petal (standard) overlaps two lateral petals (wings), which overlap the two smallest anterior petals (keel). Common in Fabaceae. Examples: Pea, Bean.

  • Androecium:

    • Stamen: Consists of a filament and an anther. Anthers are usually bilobed, and each lobe has two chambers (dithecous) called pollen sacs where pollen grains are produced.

    • Staminode: A sterile stamen.

    • Attachment:

      • Epipetalous: Stamens attached to petals. Example: Brinjal.

      • Epiphyllous: Stamens attached to the perianth. Example: Lily.

    • Cohesion:

      • Monadelphous: United into one bunch/bundle. Example: China rose.

      • Diadelphous: United into two bundles. Example: Pea.

      • Polyadelphous: United into more than two bundles. Example: Citrus.

  • Gynoecium:

    • Carpel Parts: Stigma (receptive surface for pollen), Style (elongated tube), and Ovary (enlarged basal part containing ovules on a placenta).

    • Apocarpous: Carpels are free. Examples: Lotus, Rose.

    • Syncarpous: Carpels are fused. Examples: Mustard, Tomato.

  • Placentation:

    • Marginal: Placenta forms a ridge along the ventral suture; ovules in two rows. Example: Pea.

    • Axile: Placenta is axial and ovules are attached in a multilocular ovary. Examples: China rose, Tomato, Lemon.

    • Parietal: Ovules develop on the inner wall of the ovary; ovary is one-chambered but becomes two-chambered due to a false septum called the replum. Examples: Mustard, Argemone.

    • Free Central: Ovules are borne on a central axis and septa are absent. Examples: Dianthus, Primrose.

    • Basal: Placenta develops at the base of the ovary and a single ovule is attached. Examples: Sunflower, Marigold.

The Fruit and Seed

  • The Fruit: A mature or ripened ovary developed after fertilization.

    • Parthenocarpic Fruit: Formed without fertilization; seedless. Examples: Banana, Grapes.

    • Pericarp (Fruit Wall): Can be dry or fleshy. If fleshy, it is differentiated into Epicarp (outer), Mesocarp (middle), and Endocarp (inner).

    • Drupe: Developed from monocarpellary superior ovaries and are one-seeded.

      • Mango: Epicarp is thin, mesocarp is fleshy and edible, endocarp is stony hard.

      • Coconut: Mesocarp is fibrous.

  • The Seed: Consists of a seed coat and an embryo. The embryo is made of a radicle, an embryonal axis, and one or two cotyledons.

    • Dicotyledonous Seed: Seed coat has two layers: outer testa and inner tegmen. Includes a scar called the hilum and a small pore called the micropyle.

    • Monocotyledonous Seed: Generally endospermic (except orchids). In cereals like maize, the seed coat is membranous and fused with the fruit wall.

  • Endospermic vs. Non-endospermic:

    • Endospermic (Albuminous): Endosperm is present in the mature seed. Examples: Castor (dicot), most monocots.

    • Non-endospermic (Ex-albuminous): Endosperm is consumed during development. Examples: Bean, Pea, Gram (dicots), and Orchids (monocot).