Why the Cashew Apple Is Not the Real Cashew Fruit

To a gardener, a flower is just pretty. To a botanist, every flower can be written down as a short code — how its parts are arranged, joined and counted — so that plants can be compared and identified anywhere.

This part covers inflorescence and flower structure, aestivation and placentation, floral formulae and diagrams, and fruits, seeds and family Solanaceae.

In a racemose inflorescence the main axis keeps growing and flowers form on its sides, with the oldest at the base; in a cymose inflorescence the main axis ends in a flower, with the oldest at the top; and a flower is described by its symmetry, the position of its ovary and its four whorls.

The four whorls, from outside in: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels).

Symmetry:

- Actinomorphic (radial) — mustard, datura, chilli
- Zygomorphic (bilateral) — pea, gulmohur, bean
- Asymmetric — canna

Ovary position:

- Hypogynous — ovary above other parts (superior): mustard, china rose, brinjal
- Perigynous — ovary at about the same level (half-inferior): plum, rose, peach
- Epigynous — ovary below other parts (inferior): guava, cucumber, ray florets of sunflower

An everyday example. Mustard fields in flower show racemose clusters, with fresh buds at the top and older flowers lower down.

The substance. A flower is a modified shoot, with nodes so close that its parts appear in whorls.

Aestivation is the way sepals or petals are arranged in the bud, placentation is the way ovules are arranged inside the ovary, and the androecium and gynoecium vary in how their parts are fused — cohesion for fusion within a whorl and adhesion for fusion between whorls.

Aestivation:

- Valvate — margins just touch: Calotropis
- Twisted — each overlaps the next in one direction: china rose, lady's finger, cotton
- Imbricate — overlapping without a fixed pattern: gulmohur, Cassia
- Vexillary — one large standard, two wings, two fused keel petals: pea, bean

Placentation:

- Marginal — ovules along one edge: pea
- Axile — on a central axis in a multi-chambered ovary: china rose, tomato, lemon
- Parietal — on the inner wall: mustard, Argemone
- Free central — on a central column without partitions: Dianthus
- Basal — a single ovule at the base: sunflower, marigold

Androecium. Each stamen has a filament and anther. Stamens joined to petals are epipetalous (brinjal); to tepals, epiphyllous (lily). Filaments fused into one bundle are monoadelphous (china rose), two bundles diadelphous (pea), more bundles polyadelphous (citrus).

Gynoecium. Each carpel has stigma, style and ovary; free carpels are apocarpous (lotus, rose), fused ones syncarpous (mustard, tomato).

An everyday example. Slice a tomato across and the seeds sit around a central axis in separate chambers — axile placentation.

The substance. Epipetalous stamens are an example of adhesion, since they join a different whorl.

A floral formula lists each whorl with a letter and the number of its parts — brackets for fused parts and a line under G for a superior ovary — while a floral diagram shows the same whorls as concentric circles viewed from above, with the mother axis at the top.

Symbols:

- K calyx, C corolla, P perianth, A androecium, G gynoecium
- **$(5)$ — five fused parts; $5$ — five free parts
- $\underline{G}$ — superior ovary; a line above** G shows an inferior ovary
- $\oplus$ — actinomorphic flower

Worked example — build the formula. A bisexual, actinomorphic flower has 5 fused sepals, 5 fused petals, 5 stamens attached to the petals, and 2 fused carpels in a superior ovary:

$$\oplus K_{(5)}{C}_{(5)}{A}_5\underline{G_{(2)}}$$

Floral diagram. Draw the mother axis as a dot at the top. Then draw calyx outermost, then corolla, then stamens, with the ovary in section at the centre showing its chambers and placentation. Fused parts are joined by lines.

An everyday example. An architect's floor plan shows a building from above, room by room — a floral diagram does the same for the whorls of a flower.

The substance. Numbers in brackets mean fusion, and the position of the line on G tells superior from inferior ovary — the two points most often misread.

A fruit is a ripened ovary whose wall, the pericarp, has epicarp, mesocarp and endocarp; a true fruit forms from the ovary alone and a false fruit also uses other parts; dicot seeds have two cotyledons and usually no endosperm, while monocot seeds such as maize are endospermic with a single cotyledon, the scutellum.

Fruits. In a mango, the epicarp is the skin, the mesocarp the juicy edible pulp and the endocarp the hard stone. Apple and cashew are false fruits, since the thalamus or stalk forms the fleshy part.

Dicot seed (gram, bean). Seed coat with testa and tegmen; hilum and micropyle; an embryonal axis with plumule and radicle between two cotyledons.

Monocot seed (maize). Seed coat fused with the fruit wall; a large endosperm separated from the embryo by the protein-rich aleurone layer; one shield-shaped cotyledon, the scutellum; plumule sheathed by the coleoptile and radicle by the coleorhiza.

Family Solanaceae — semi-technical description:

- Vegetative: herbs or shrubs; leaves alternate, simple, reticulate; potato has an underground stem tuber
- Flower: bisexual, actinomorphic
- Calyx: 5 united, persistent; corolla: 5 united
- Androecium: 5 stamens, epipetalous
- Gynoecium: bicarpellary, syncarpous, superior ovary, axile placentation
- Fruit: berry or capsule; many endospermic seeds

An everyday example. Tomato, brinjal, potato and chilli in an Indian vegetable basket all belong to one family, Solanaceae.

The substance. The calyx often stays attached to a Solanaceae fruit — look at the green cap on a brinjal.