Comprehensive Morphology and Structural Organisation of Flowering Plants
Introduction to Biological Description and Reductionist Biology
Until the establishment of experimental biology and physiology, biology was primarily a natural history focused on the description of diverse life forms through observation with the naked eye, magnifying lenses, and microscopes.
Description Scope: Early biological descriptions focused on gross structural features, both external (morphology) and internal (anatomy), as well as observable living phenomena.
Significance of Detailed Description: While students may initially perceive detailed structural descriptions as tedious, they serve as the foundational data for later reductionist biology, where the focus shifted to living processes and physiological mechanisms.
Application: Structural descriptions provide the necessary framework for asking meaningful research questions in fields such as physiology and evolutionary biology.
Unit Overview: This unit covers the structural organization of plants and animals, presenting morphological and anatomical features separately for convenience. The chapters include:
Chapter : Morphology of Flowering Plants
Chapter : Anatomy of Flowering Plants
Chapter : Structural Organisation in Animals
Biography of Katherine Esau ( – )
Early Life and Education: Katherine Esau was born in Ukraine in . She studied agriculture in Russia and Germany before moving to the United States, where she earned her doctorate in .
Scientific Contributions:
Her early publications established that the curly top virus is transported through a plant via the phloem (food-conducting) tissue.
In , she published Plant Anatomy, which utilized a dynamic and developmental approach to plant structure. This work had a worldwide impact and revived the discipline.
In , she published The Anatomy of Seed Plants, a work widely regarded as the "Webster’s of plant biology" due to its encyclopedic nature.
Awards and Recognition:
Elected to the National Academy of Sciences in (the sixth woman to be honored).
Awarded the National Medal of Science by President George Bush in .
Legacy: Peter Raven, director of Anatomy and Morphology at the Missouri Botanical Garden, noted that Esau "absolutely dominated" the field of plant biology even at the age of .
Morphology of Flowering Plants: The Root System
General Characteristics: Most higher plants (angiosperms) possess roots, stems, leaves, flowers, and fruits. The underground portion is the root system, while the aerial portion is the shoot system.
Root Types:
Tap Root System: Formed by the direct elongation of the radicle into a primary root. It bears lateral roots of different orders (, , etc.). This is characteristic of dicotyledonous plants, such as the mustard plant.
Fibrous Root System: Common in monocotyledonous plants (e.g., wheat) where the primary root is short-lived and replaced by numerous roots originating from the base of the stem.
Adventitious Roots: Roots that develop from parts of the plant other than the radicle, appearing in plants like grass, Monstera, and the banyan tree.
Main Functions of Roots:
Absorption of water and minerals from the soil.
Providing proper anchorage to plant parts.
Storing reserve food material.
Synthesis of plant growth regulators.
Regions of the Root:
Root Cap: A thimble-like structure at the apex that protects the tender root tip as it pushes through soil.
Region of Meristematic Activity: Located a few millimeters above the root cap; contains very small, thin-walled cells with dense protoplasm that divide repeatedly.
Region of Elongation: Proximal to the meristematic zone; cells here undergo rapid enlargement, increasing the root's length.
Region of Maturation: Proximal to the elongation zone; cells differentiate and mature. Epidermal cells in this region form fine, delicate root hairs for water and mineral absorption.
Root Modifications:
Storage: Roots become swollen to store food, such as the tap roots of carrot and turnip, or the adventitious roots of sweet potato and asparagus.
Mechanical Support:
Prop roots: Hanging structures in banyan trees.
Stilt roots: Roots emerging from lower stem nodes in maize and sugarcane.
Respiration: In plants like Rhizophora (growing in swampy areas), specialized roots called pneumatophores grow vertically upward to obtain oxygen from the air.
The Shoot System: The Stem
General Characteristics: The stem is the ascending part of the plant axis, developing from the plumule of the embryo. It bears nodes (where leaves are born) and internodes (the segments between nodes).
Features: Stems bear terminal or axillary buds. Stems are usually green when young and may become woody and dark brown later in life.
Functions: Branching out for leaves/flowers/fruits; conduction of water, minerals, and photosynthates; storage, support, protection, and vegetative propagation.
Stem Modifications:
Storage: Underground stems of potato, ginger, turmeric, zaminkand, and Colocasia. These also serve as organs of perennation to survive unfavorable growth conditions.
Support: Stem tendrils (slender, spirally coiled) develop from axillary buds to help climbing in gourds (cucumber, pumpkin, watermelon) and grapevines.
Protection: Axillary buds may modify into woody, straight, pointed thorns (e.g., Citrus, Bougainvillea) to protect against browsing animals.
Photosynthesis: In arid regions, stems become flattened (Opuntia) or fleshy cylindrical (Euphorbia). They contain chlorophyll and perform photosynthesis.
Spread and Vegetative Propagation:
Underground stems of grass and strawberry spread to new niches.
Lateral branches in mint and jasmine grow aerially then arch to the ground to root.
Aquatic plants like Pistia and Eichhornia have lateral branches with short internodes and leaf rosettes at nodes.
In banana, pineapple, and Chrysanthemum, lateral branches grow horizontally underground and then emerge obliquely upward.
The Shoot System: The Leaf
Characterization: The leaf is a lateral, flattened structure originating from shoot apical meristems and arranged in an acropetal order. It is the primary site of photosynthesis.
Parts of a Leaf:
Leaf Base: Attaches the leaf to the stem. In monocots, it may sheath the stem. In legumes, it may become swollen, known as the pulvinus. It may bear small leaf-like structures called stipules.
Petiole: Holds the lamina to light. Flexible petioles allow fluttering, cooling the leaf and circulating air.
Lamina (Leaf Blade): The expanded green part containing veins and veinlets, with a prominent midrib.
Venation:
Reticulate: Veinlets form a network; common in dicots.
Parallel: Veins run parallel to each other; common in monocots.
Types of Leaves:
Simple: Lamina is entire or incisions do not reach the midrib.
Compound: Incisions reach the midrib, dividing the lamina into leaflets. A bud is present in the axil of the petiole, but never in the axil of leaflets.
Pinnately Compound: Leaflets on a common axis (rachis), e.g., neem.
Palmately Compound: Leaflets attached at a common point (tip of petiole), e.g., silk cotton.
Phyllotaxy (Arrangement Patterns):
Alternate: A single leaf at each node, e.g., China rose, mustard, sunflower.
Opposite: A pair of leaves at each node, e.g., Calotropis, guava.
Whorled: More than two leaves at a node forming a whorl, e.g., Alstonia.
Leaf Modifications:
Climbing: Tendrils (e.g., peas).
Defense: Spines (e.g., cacti).
Storage: Fleshy leaves (e.g., onion, garlic).
Photosynthesis (Phyllode): Expanded green petioles in Australian acacia.
Insectivory: Modified leaves in pitcher plants and Venus flytraps.
Inflorescence
Definition: The arrangement of flowers on the floral axis. A flower itself is a modified shoot where the apical meristem transforms into a floral meristem.
Types:
Racemose: The main axis continues to grow; flowers are produced laterally in acropetal succession.
Cymose: The main axis terminates in a flower, limiting growth; flowers are born in basipetal order.
The Flower: Reproductive Unit of Angiosperms
Structure: A typical flower consists of four whorls on a swollen stalk end called the thalamus (receptacle):
Accessory Organs: Calyx (sepals) and Corolla (petals).
Reproductive Organs: Androecium (stamens) and Gynoecium (carpels).
Perianth: Term used when calyx and corolla are not distinct (e.g., lily).
Symmetry:
Actinomorphic: Radial symmetry (e.g., mustard, datura, chilli).
Zygomorphic: Bilateral symmetry (e.g., pea, gulmohur, bean, Cassia).
Asymmetric: No vertical plane of symmetry (e.g., Canna).
Floral Parts and Arrangement:
Meristy: Trimerous (), tetramerous (), or pentamerous ().
Bracts: Reduced leaves at the pedicel base; categorized as bracteate or ebracteate.
Ovary Position:
Hypogynous: Gynoecium is highest; superior ovary (e.g., mustard, China rose, brinjal).
Perigynous: Gynoecium central, other parts on the thalamus rim; half-inferior ovary (e.g., plum, rose, peach).
Epigynous: Thalamus encloses ovary completely; inferior ovary (e.g., guava, cucumber, ray florets of sunflower).
Calyx and Corolla Characteristics:
Calyx: Sepals can be gamosepalous (united) or polysepalous (free).
Corolla: Petals can be gamopetalous (united) or polypetalous (free). Shapes include tubular, bell-shaped, funnel-shaped, or wheel-shaped.
Aestivation (Arrangement of petals/sepals):
Valvate: Margins just touch without overlapping (e.g., Calotropis).
Twisted: One margin overlaps the next (e.g., China rose, lady's finger, cotton).
Imbricate: Overlapping without a specific direction (e.g., Cassia, gulmohur).
Vexillary (Papilionaceous): Five petals: one large standard, two lateral wings, two anterior keels (e.g., pea, bean).
Androecium:
Composed of stamens (filament and anther). Anthers are usually bilobed with two pollen sacs each.
Staminode: A sterile stamen.
Attachment: Epipetalous (to petals, e.g., brinjal) or epiphyllous (to perianth, e.g., lily).
Fusion: Monadelphous (one bundle, e.g., China rose), diadelphous (two bundles, e.g., pea), or polyadelphous (multiple bundles, e.g., citrus).
Gynoecium:
Composed of carpels (stigma, style, ovary).
Apocarpous: Carpels are free (e.g., lotus, rose).
Syncarpous: Carpels are fused (e.g., mustard, tomato).
Placentation (Arrangement of ovules):
Marginal: Ventral suture ridge (e.g., pea).
Axile: Multilocular ovary with central axis (e.g., China rose, tomato, lemon).
Parietal: Inner wall/peripheral part; may appear two-chambered due to false septum (e.g., mustard, Argemone).
Free Central: Central axis without septa (e.g., Dianthus, primrose).
Basal: Base of ovary, single ovule (e.g., sunflower, marigold).
The Fruit and The Seed
The Fruit: A ripened ovary developed after fertilization. Parthenocarpic fruits form without fertilization.
Pericarp: The fruit wall, differentiated into epicarp (outer), mesocarp (middle), and endocarp (inner).
Drupe: Fleshy fruit with a stony endocarp. Examples: Mango (fleshy edible mesocarp) and Coconut (fibrous mesocarp).
The Seed: Developed from ovules.
Dicotyledonous Seed: Contains seed coat (testa and tegmen), hilum (attachment scar), micropyle (pore), embryonal axis with a radicle and plumule, and two cotyledons.
Endospermous: e.g., castor.
Non-endospermous: e.g., bean, gram, pea.
Monocotyledonous Seed: Generally endospermic (except orchids).
Structure: Seed coat fused with fruit wall (cereals). Endosperm is separated by a proteinous aleurone layer.
Embryo: Specific parts include the scutellum (shield-shaped cotyledon), coleoptile (plumule sheath), and coleorhiza (radicle sheath).
Systematic Semi-Technical Description of Families
Floral Formula Symbols:
: Bracteate
: Calyx
: Corolla
: Perianth
: Androecium
: Gynoecium
: Superior ovary
: Inferior ovary
: Male
: Female
: Bisexual
: Actinomorphic
: Zygomorphic
Fabaceae (formerly Papilionoideae):
Vegetative: Trees/shrubs/herbs with root nodules; alternate, pinnately compound leaves with pulvinate leaf base.
Floral: Racemose inflorescence; zygomorphic, bisexual; petals five (papilionaceous); androecium ten (diadelphous).
Formula:
Economic Importance: Pulses (gram, arhar, moong), edible oil (soybean, groundnut), dye (Indigofera), fibers (sunhemp), fodder (Sesbania, Trifolium), ornamentals (lupin, sweet pea), and medicine (muliathi).
Solanaceae (Potato family):
Vegetative: Mostly herbs/shrubs; underground stem in potato (Solanum tuberosum); alternate, simple leaves.
Floral: Cymose inflorescence; actinomorphic, bisexual; five united sepals (persistent) and petals; five epipetalous stamens; bicarpellary syncarpous superior ovary.
Formula:
Economic Importance: Food (tomato, brinjal, potato), spice (chilli), medicine (belladonna, ashwagandha), fumigatory (tobacco), and ornamentals (petunia).
Liliaceae (Lily family):
Vegetative: Perennial herbs with bulbs/corms/rhizomes; basal, linear leaves with parallel venation.
Floral: Umbellate clusters; bisexual, actinomorphic; perianth six () united tepals; stamens six () epitepalous; tricarpellary syncarpous trilocular superior ovary with axile placentation.
Formula:
Economic Importance: Ornamentals (tulip, Gloriosa), medicine (Aloe), vegetables (Asparagus), and colchicine (Colchicum autumnale).