Comparative Floral Morphometrics: Floral Diagrams & Formulas of Major Angiosperm Families
In angiosperm taxonomy, the flower represents a determinate reproductive shoot bearing modified leaves in specialized whorls. Because reproductive floral structures are subject to strong stabilizing selection for pollinator specificity, floral characters are far more conserved and taxonomically informative than vegetative organs.
Floral Diagram Principles: A floral diagram is a schematic projection of a cross-section of a floral bud relative to the mother axis (represented by a black dot at the top, denoting posterior orientation):
1. Calyx (Sepals, K): The outermost whorl, drawn with an outer ridge or dorsal keel.
2. Corolla (Petals, C): Alternating with sepals, drawn as smooth arcs.
3. Androecium (Stamens, A): Shown as bilobed transverse sections of anthers facing inwards (introrse) or outwards (extrorse).
4. Gynoecium (Carpels, G): Positioned at the exact center, showing ovary locules, placentation type, and ovule attachment.
5. Fusion Symbols: Cohesion (fusion of parts within the same whorl, e.g. gamopetaly) is shown by connecting lines. Adnation (fusion between different whorls, e.g. epipetaly) is shown by radial connecting bridges or square brackets.
Comparative Family Features:
• Brassicaceae (Cruciferae): Actinomorphic ($oplus$), $K_{2+2}$ (2 lateral saccate + 2 anteroposterior), $C_4$ cruciform (cross-like), $A_{2+4}$ tetradynamous (2 short outer lateral stamens and 4 long inner median stamens), $underline{G}_{(2)}$ bicarpellary syncarpous with parietal placentation and a persistent false septum termed the replum.
• Fabaceae (Papilionoideae): Zygomorphic ($%$), $K_{(5)}$ gamosepalous, $C_{1+2+(2)}$ papilionaceous corolla (1 large posterior standard/vexillum, 2 lateral wings/alae, 2 anterior fused keel/carina petals), $A_{(9)+1}$ diadelphous (9 stamens united into an open sheath + 1 posterior stamen free), $underline{G}_1$ monocarpellary with marginal placentation.
• Solanaceae (Nightshade): Actinomorphic ($oplus$), pentamerous, $K_{(5)}$ persistent and often accrescent, $C_{(5)}$ rotate/campanulate, $[C_{(5)} A_5]$ epipetalous stamens with alternating insertion, $underline{G}_{(2)}$ bicarpellary with an oblique ovary septum placed at $35^circ–45^circ$ to the anteroposterior axis and swollen axile placentae bearing numerous ovules.
- Fresh Inflorescences: Fresh flowering twigs of *Brassica campestris* (Mustard), *Pisum sativum* (Pea), and *Solanum nigrum* (Black nightshade).
- Dissection Tools: Fine watchmaker’s forceps (No. 5), micro-dissection needles, razor blades for transverse ovary sectioning, camel-hair brush.
- Optics: Stereo zoom dissection microscope ($10times–40times$) and standard compound light microscope.
- Glassware: Glass Petri dishes with black wax bottoms, microscope slides, and water droplets for ovary mounts.
Record inflorescence architecture (e.g., corymbose raceme in *Brassica*, solitary axillary/racemose in *Pisum*, extra-axillary cyme in *Solanum*). Note presence or absence of bracts (Bracteate vs Ebracteate) and pedicel length.
Strip sepals and petals sequentially from the outside under the dissection microscope. Determine symmetry: dissect down the median plane to confirm zygomorphy in *Pisum* vs radial actinomorphy in *Brassica* and *Solanum*. Record aestivation (valvate, imbricate, or vexillary). Note any cohesion (fused petals in *Solanum*).
Count stamen number and observe insertion (hypogynous vs epipetalous). In *Brassica*, measure filament lengths with an ocular micrometer to verify tetradynamy ($2text{ short} + 4text{ long}$). In *Pisum*, confirm diadelphous condition: 9 filaments united into a basal trough and 1 posterior free filament.
Excise the gynoecium. Cut an ultra-thin transverse section (T.S.) of the ovary with a clean razor blade. Mount in water on a slide and examine under $100times$. Identify: locule count, septum orientation, placentation type (parietal with replum in *Brassica*, marginal in *Pisum*, axile with oblique septum and swollen placenta in *Solanum*).
| Character | Brassicaceae (*Brassica*) | Fabaceae (*Pisum*) | Solanaceae (*Solanum*) |
|---|---|---|---|
| Floral Symmetry | Actinomorphic (⊕) | Zygomorphic (%) | Actinomorphic (⊕) |
| Calyx (K) | K₂⁺₂, polysepalous, 2 saccate | K(₅), gamosepalous, valvate | K(₅), persistent, accrescent |
| Corolla (C) | C₄, polypetalous, cruciform | C₁⁺₂⁺(₂), papilionaceous (vexillary) | C(₅), gamopetalous, rotate/tubular |
| Androecium (A) | A₂⁺₄, tetradynamous | A(₉)⁺₁, diadelphous | [C(₅) A₅], epipetalous, alternipetalous |
| Gynoecium (G) | |||
| Placentation | Parietal (with false replum) | Marginal | Axile (swollen placenta) |
| Diagnostic Fruit | Siliqua or Silicula | Legume / Pod | Berry or Septicidal Capsule |
| Standard Formula | ⊕ ♂♀ K₂⁺₂ C₄ A₂⁺₄ | % ♂♀ K(₅) C₁⁺₂⁺(₂) A(₉)⁺₁ | ⊕ ♂♀ K(₅) [C(₅) A₅] |
Taxonomic Synthesis:
The floral morphometric data captures distinct evolutionary adaptations: specialized bilateral symmetry (%) and explosive diadelphous pollen release in Fabaceae for bee pollination (hymenopteran co-evolution), versus generalized radial actinomorphy with nectar glands at stamen bases in Brassicaceae, and fused epipetalous corolla tubes with an oblique ovary in Solanaceae.
