The Planters' Guild
Module I · How a Plant Works
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Lesson 6 of 10 · ~7 min

Inside a leaf

A solar collector with adjustable pores and a factory floor in the middle.

The leaf is the plant's solar collector: flat, broad, held into the air to catch light while exchanging gases and not losing too much water. Almost every visible leaf trait is a tradeoff among those three.

The layers

Top to bottom: a waxy cuticle (waterproofing), a transparent upper epidermis (the leaf's outer skin of cells), the palisade mesophyll , the spongy mesophyll, the vascular bundles (veins, bringing water in and sugar out), and a lower epidermis where most stomata sit.

The palisade layer is tightly packed and dense with chloroplasts, the green compartments inside a cell where photosynthesis happens. The spongy layer below it is loose, with air spaces for gas to circulate.

Labeled leaf cross-section: cuticle and upper epidermis on top, palisade mesophyll, spongy mesophyll, a central vascular bundle, and a guard-cell inset. Cuticle Upper epidermis Palisade mesophyll Spongy mesophyll Vascular bundle Guard cells

Fig. 7. Light enters through the clear cuticle and epidermis; the palisade layer does most photosynthesis; spongy mesophyll handles gas flow; veins deliver water and remove sugar; stomata below manage gas exchange.

Stomata : the adjustable pores

A stoma is a tiny pore formed by two guard cells, shaped like a pair of curved sausages. When they take in water and swell, their shape and the stiffening of their walls make them bow apart, opening the pore between them; when they lose water they slacken and the pore closes. Essentially hydraulic. (Exactly which part of the wall does the stiffening is still argued over, so don't let anyone tell you it's settled; see Carter et al., 2017.)

Light opens them, to take in CO2. What closes them is water stress: dry air pulling moisture out of the leaf faster than the roots can replace it, or dry substrate signaling the plant to shut down (Hetherington & Woodward, 2003). Warmth on its own does the opposite of what most people assume. A warm leaf with plenty of water tends to open its stomata wider, because evaporation is how it cools itself (Urban et al., 2017). Once the stomata do shut, CO2 can no longer get in and photosynthesis falls even under bright light (Lawson & Blatt, 2014). That's why a plant in a sunny, bone-dry room can be flooded with light and still barely growing, and why the fix is humidity and water rather than shade.

Labeled guard cells: open pore on the left, closed on the right, with a guard cell and the stoma marked. Open Closed Guard cell Stoma

Fig. 8. Guard cells open (left) when full of water, closed (right) when not. The plant trades water loss for CO2 every time it opens.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

It's a blazing, dry afternoon. Your plant is in bright light but seems to be doing nothing. What's happening inside the leaf, and what would actually help?
The stomata have closed to conserve water, so CO2 can't get in. There's plenty of light, but with the gas door shut the plant is idling. Note what's driving it: the dry air, not the warmth. Raising humidity or watering helps; cooling the room by itself doesn't.
Connect it back to Lesson 5: those same closed stomata also slow which process?
Transpiration. Closing the stomata cuts water loss, which also eases the tension on the water column you drove in the simulation.
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