Unit 1: Angiosperm Morphology and Taxonomy

In the study of plant morphology, the leaf is a vital vegetative organ. This section focuses on how leaves are arranged and how they change to perform specialized functions.

Leaf Phyllotaxy

Phyllotaxy refers to the pattern of arrangement of leaves on the stem or branch.

• Alternate: A single leaf arises at each node in an alternating manner (e.g., Hibiscus).
• Opposite: A pair of leaves arises at each node opposite to each other. This can be Opposite Decussate (pairs at right angles, e.g., Calotropis) or Opposite Superposed (pairs in the same plane, e.g., Guava).
• Whorled: More than two leaves arise at a single node forming a circle or whorl (e.g., Nerium).

Leaf Modifications

Leaves often modify their structure to adapt to specific environmental needs or to perform non-photosynthetic functions.

• Leaf Tendrils: Whole leaf or parts of it modify into thread-like structures for climbing (e.g., Pea).
• Leaf Spines: Leaves modify into sharp, pointed structures for defense and to reduce water loss (e.g., Aloe, Cactus).
• Phyllode: The petiole becomes green, flat, and leaf-like to perform photosynthesis when the true leaves are reduced (e.g., Australian Acacia).
• Storage Leaves: Fleshy leaves that store food or water (e.g., Onion, Garlic).

Inflorescence is the arrangement of flowers on the floral axis. While many follow simple racemose or cymose patterns, certain groups exhibit specialized forms.

Special Inflorescence Types

• Cyathium: Characteristic of Euphorbia. It looks like a single flower but is actually a collection of many male flowers (reduced to single stamens) and one central female flower (a single tricarpellary pistil) enclosed in a cup-shaped involucre.
• Verticillaster: Characteristic of the family Lamiaceae. It consists of two sessile cymes produced at each node in the axils of opposite leaves, appearing like a false whorl of flowers around the stem.
• Hypanthodium: Characteristic of the genus Ficus (e.g., Banyan, Peepal). The fleshy receptacle forms a hollow, flask-shaped cavity with a small opening (ostiole) at the top. Small flowers are borne on the inner wall of the cavity.

As per the syllabus, the systematic position of plants must be studied according to Bentham and Hooker's system of classification.

Outline of Bentham & Hooker's System

This is a Natural System based on the observation of a large number of characters. The Division Phanerogams (seed plants) is divided into three classes:

1. Dicotyledons: Seeds with two cotyledons; leaves with reticulate venation. Divided into Polypetalae, Gamopetalae, and Monochlamydeae.
2. Gymnospermae: Seeds are naked (not enclosed in an ovary).
3. Monocotyledons: Seeds with one cotyledon; leaves with parallel venation.

This system is highly practical for herbarium organization and is widely followed in India and Britain.

The course requires a detailed study of the vegetative and floral characters of the following families:

A. Fabaceae (Legume Family)

• Vegetative: Trees, shrubs, or herbs; roots with nodules containing Nitrogen-fixing bacteria.
• Floral: Zygomorphic, papilionaceous corolla (standard, two wings, and two keels).
• Gynoecium: Monocarpellary, superior ovary with marginal placentation.
• Formula: % K(5) C1+2+(2) A(9)+1 G1

B. Solanaceae (Potato Family)

• Vegetative: Mostly herbs or shrubs; stem often with bicollateral vascular bundles.
• Floral: Actinomorphic, pentamerous; Persistent Calyx (stays with the fruit).
• Gynoecium: Bicarpellary, syncarpous, superior ovary with swollen placenta.
• Formula: + K(5) [C(5) A5] G(2)

C. Lamiaceae (Mint Family)

• Vegetative: Square stems; aromatic leaves with oil glands; opposite decussate phyllotaxy.
• Floral: Verticillaster inflorescence; bilabiate (two-lipped) corolla.
• Gynoecium: Gynobasic style; ovary 4-lobed.

D. Euphorbiaceae (Spurge Family)

• Vegetative: Often contains milky latex; highly variable habits.
• Floral: Cyathium inflorescence (in some); flowers are always unisexual.
• Gynoecium: Tricarpellary, syncarpous, superior ovary.

E. Poaceae (Grass Family)

• Vegetative: Cylindrical, hollow stems (culms); leaves with sheathing bases and ligules.
• Floral: Inflorescence is a spikelet; flowers are called florets, enclosed by glumes, lemma, and palea.
• Gynoecium: Monocarpellary or tricarpellary (appearing monocarpellary), superior ovary with feathery stigmas.

5. Exam Focus: Practical Tips & Viva Prep

Viva Questions

• What is a gynobasic style? A style that arises from the base of the ovary (characteristic of Lamiaceae).
• How do you identify a Cyathium? Look for a cup-shaped structure containing a central "stalked" female flower and multiple male flowers consisting of only one stamen.
• What is the systematic position of Fabaceae? Class: Dicotyledonae, Subclass: Polypetalae, Series: Calyciflorae, Order: Rosales (according to Bentham & Hooker).

Common Pitfalls

• Misidentifying Inflorescence: Confusing the Cyathium with a single flower because of its compact structure.
• Formula Errors: Forgetting the brackets around numbers in floral formulas (brackets indicate fusion/syncarpy/gamopetaly).