Module 0: Floral Anatomy & Morphology

Goethe's 1790 essay Die Metamorphose der Pflanzen argued that every floral organ is a modified leaf. Modern developmental genetics (Module 1) vindicates this view: carpels, stamens, petals and sepals all derive from a common leaf-like ground state whose identity is specified by overlapping domains of MADS-box transcription factors. Proceeding from the outside inward along the floral axis, the four whorls are:

1.1 The Perianth

Calyx + corolla together form the perianth. When sepals and petals are indistinguishable (as in many monocots), the individual organs are called tepals. The perianth is the visible part of the flower in most horticultural species and it is what we usually think of as "the flower."

1.2 Cross-Section of a Generic Flower

Below is a longitudinal section of a typical hermaphrodite eudicot flower (e.g. Rosa or Prunus), showing the relationship of the four whorls, the receptacle, and the vascular supply.

1.3 Ovary Position

Three configurations describe where the other floral parts are attached relative to the ovary:

A floral formula is a compact one-line description. Each whorl is denoted by a letter, followed by the number of organs (or ∞ for many, or a range):

Parentheses indicate fusion of organs within a whorl (\(\text{C}_{(5)}\)= 5 fused petals); square brackets indicate fusion between whorls (\([\text{C}_5\text{A}_5]\) = corolla and androecium fused). A prefixed asterisk denotes actinomorphy (*), a zigzag for zygomorphy (↯), and the sex symbol for hermaphrodite (☿) or unisexual flowers.

2.1 Floral Diagram Conventions

A floral diagram is a cross-section (looking down the floral axis) that preserves the relative positions of the whorls. The floral axis (the stem) is marked at the top; the subtending bract, if present, at the bottom. Each organ is drawn with a standard symbol:

The single largest morphological split among flowering plants is between monocots (Monocotyledoneae, ~70 000 species) and eudicots (Eudicotyledoneae, ~175 000 species). Their flowers differ in a stereotyped way:

3.1 Symmetry

Flowers are classified by the number of planes of bilateral symmetry:

• Actinomorphic (radial): multiple planes of symmetry (infinite for a perfect radial flower). Typical of wind-pollinated and generalist bee-pollinated flowers: buttercup, tulip, apple blossom.
• Zygomorphic (bilateral): a single plane of symmetry. Associated with specialised pollinators requiring a landing platform: orchids, Fabaceae (pea family), Lamiaceae (mints).
• Asymmetric: no planes of symmetry; rare (Cannaceae).

3.2 Merosity and Fibonacci

The number of organs in a whorl is the merosity. Across angiosperms the most common values are 3 (monocots), 4 (Brassicaceae), and 5 (most eudicots). These are all Fibonacci numbers (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...). Phyllotaxis produces Fibonacci spirals because the golden angle \(\psi = 2\pi(1-1/\varphi) \approx 137.508^\circ\)is the irrational number that is "most difficult" to approximate by rationals (Hurwitz's theorem): any approximation \(|x - p/q| < 1/(\sqrt{5}\,q^2)\) is saturated by golden-ratio convergents. Consecutive primordia in a golden-angle pattern interleave optimally and never shadow each other along radial lines.

The resulting spiral has two families of parastichies whose counts are consecutive Fibonacci numbers (e.g. 34 and 55 in a small sunflower head, 89 and 144 in a large one). The simulation below generates a Vogel spiral and illustrates what happens if the divergence angle departs from \(\psi\).

Few species bear solitary flowers. Most plants produce clusters called inflorescences. The branching pattern is diagnostic at the family level. The two fundamental categories are indeterminate (racemose) and determinate (cymose):

4.1 The Asteraceae Capitulum

Sunflowers, daisies and dandelions combine hundreds of tiny flowers (florets) into a single capitulum that behaves as one functional flower for pollinators. Two floret types coexist:

• Disc florets - actinomorphic, tubular, in the centre; usually bisexual.
• Ray florets - zygomorphic, strap-shaped, at the margin; usually sterile or female-only.

The capitulum represents convergent evolution with orchids: thousands of independent events packaged as one advertising unit. The florets open in centripetal waves so that the display lasts days, maximizing total pollinator visits (analogy to collective foraging strategies).

Each floral organ is supplied by one or more vascular bundles. The traces depart from the central stele of the pedicel and pass through the receptacle into the organ. A typical pentamerous flower has five groups of traces, each serving one sepal, its petal, the opposite stamen whorl, and part of the carpel wall.

Water supply to petals relies on phloem as well as xylem: petals have low stomatal density and are osmotically pulled turgid by phloem-delivered sucrose. This explains why cut flowers last longer in sucrose solutions ("flower food"): the petals continue to osmose water. The Poiseuille relation governs transport through the pedicel's xylem:

5.1 Thermogenesis in Flowers

A handful of plant families (Araceae, Nymphaeaceae, Nelumbonaceae, Magnoliaceae, Cycadaceae, Annonaceae) produce flowers that actively warm themselves during anthesis, raising tissue temperature by 10-30 °C above ambient. Classic examples:

• Symplocarpus foetidus (skunk cabbage) - melts snow, constant 15-25 °C for ~14 days.
• Philodendron selloum - maintains 46 °C in air as cold as 4 °C, a homeothermy unique in plants.
• Nelumbo nucifera (sacred lotus) - thermoregulates receptacle to 30-35 °C; attracts heat-seeking beetles.
• Amorphophallus titanum (titan arum) - corpse flower; 36 °C peak for ~2 days, volatilising carrion odours.

5.2 Mechanism: Alternative Oxidase (AOX)

Standard mitochondrial respiration couples electron transport to ATP synthesis through the electrochemical proton gradient at Complex V. In thermogenic flowers, a second ubiquinone-oxidising enzyme - alternative oxidase, encoded by the AOX1 gene - short-circuits the chain:

Because AOX bypasses Complexes III and IV, no proton gradient is built - the redox free energy released is dissipated as heat. AOX is cyanide-insensitive and salicylic-acid inducible. The power output per mitochondrion is enormous: up to 0.4 W/g fresh tissue in Arum, comparable to a hovering hummingbird's pectoralis. The runs down a starch reserve:

5.3 Thermoregulation Simulation

Below is a lumped-parameter model of the Philodendron spadix: heat production by AOX-mediated respiration balanced by convective loss to ambient air and Stefan-Boltzmann radiation. Starch reserves (parameterised by a finite energy pool \(E_0\)) cap the duration of the burst; temperature dependence follows a \(Q_{10}\) law\(Q(T) = Q_0 Q_{10}^{(T-T_\text{ref})/10}\).

The peak \(\Delta T\) of 20-30 °C matches published measurements on Philodendron selloum (Nagy et al. 1972). Integrating the heat curve gives a total energy expenditure of a few kJ - supplied by 100-200 mg of starch, which is about 5% of the spadix fresh mass. The price of being a furnace is high.