Chapter two

Anatomy of a seed

A bean is an entire plant, folded up and dehydrated, wrapped in a waterproof case with a one-way valve. Nine named structures, and almost all of the mass is in two of them.

Red kidney bean

Drag to rotate, scroll to zoom, tap a marker for detail. Cells shows starch granules (cream, 10–45 µm in life) and protein storage vacuoles (amber, 2–20 µm) packed inside the cotyledon.

Figure 2.1

Section, with the walls magnified

The same geometry as the model above, sliced along the plane of symmetry. Both callouts are drawn to their real relative scale: the seed coat is roughly 70 µm thick, a cotyledon cell about 100 µm across.

Left callout — the testa. An outer cuticle over a palisade of macrosclereids with heavily thickened radial walls, then a course of hourglass-shaped osteosclereids, then loose parenchyma. The palisade is what makes a hard seed impermeable; it is also where the tannins and anthocyanins sit. Right callout — a cotyledon cell. Starch granules and protein storage vacuoles packed against a pectin-rich wall. Whether a cooked bean is creamy or gritty is decided by whether those walls separate or rupture.

Figure 2.2

Two ways out of the ground

The common bean lifts its cotyledons into the light on an arching hypocotyl. The runner bean, pea and fava leave theirs buried and push up a shoot instead. Scrub the slider to run both.

Germination

Day 0 — dry seed

Epigeal — Phaseolus vulgaris. The hypocotyl elongates and hooks, dragging the cotyledons up. They green briefly, feed the seedling for about two weeks, then drop.

Hypogeal — Pisum sativum, Vicia faba, P. coccineus. The epicotyl elongates instead; the cotyledons stay below ground. Costlier in reserves, but the growing point is protected from frost and grazing.

0 d

Figure 2.3

Breaking the strongest bond in the air

Dinitrogen is 78% of the atmosphere and almost inert: the N≡N triple bond takes 941 kJ/mol to break. Industry does it at 450 °C and 200 atmospheres. A bean does it in a root nodule at ambient temperature, and pays in ATP.

Symbiosis, step by step

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The reaction

N₂ + 8H⁺ + 8e⁻ + 16 ATP
→ 2NH₃ + H₂ + 16 ADP + 16 P_i

Note the obligatory hydrogen. Nitrogenase cannot reduce N₂ without also making H₂, so at least 25% of the electron flux is spent on a by-product. Sixteen ATP per molecule of nitrogen is the going rate, and the plant pays it out of photosynthesis — a nodulated legume spends 4–16% of its fixed carbon on the bacteria.

How much

Nitrogen fixed per hectare per season

Typical field ranges. The common bean is a famously poor fixer for a legume — one reason it is usually grown with some fertiliser and why breeding programmes target its symbiotic efficiency.

Figure 2.4

Where beans sit in the family

Fabaceae is the third-largest family of flowering plants and the only one that reorganised its own higher taxonomy this century — the 2017 Legume Phylogeny Working Group classification split the old catch-all Caesalpinioideae into four.

Six subfamilies

LPWG 2017

Down to the bean

Figure 2.5

The pod is the whole point

Everything that makes a legume a legume follows from one organ. A pod — a legume in the strict botanical sense — is a dry fruit developed from a single carpel that splits along two seams at once. Nothing else in the plant kingdom does that.

Why two seams matters

Dehiscing along both sutures throws seed clear of the parent — the pod halves twist as they dry and flick the seeds metres away. The single most important thing done to a wild legume during domestication was to break that mechanism, so the crop holds its seed until harvest. Almost every pulse carries a loss-of-function mutation in pod shattering.

Placentation

Seeds attach alternately along one suture, each by a funiculus that leaves the hilum scar. Count the hila along a split pod and you can read off how many ovules were fertilised and how many aborted — a bean pod typically sets four to six of eight.

The flower it came from

Papilionaceous: a broad standard petal at the back, two wings at the sides, and two lower petals fused into a keel that encloses ten stamens and the ovary. A visiting bee levers the keel down and trips the stigma against its abdomen. Most cultivated beans, though, gave that up and pollinate themselves before the flower even opens.

Not every pod is dry

Tamarind and carob keep theirs fleshy and indehiscent — sugary pods that pay animals to carry the seed instead of throwing it. Both sit outside Faboideae, and neither is what anyone means by a bean.