Leaves: Mesophyll, Stomata, Venation and Adaptations

Solar panels that can open and close

A leaf has a tricky job. To make sugar, it needs carbon dioxide from the air, so it must let air in. But every opening that lets CO₂ in also lets water vapour out. A plant that opens up too much dries out; a plant that closes up too much starves. Leaves solve this problem with clever structure, and different plants solve it in different ways, from the needles of a black spruce in a Yukon winter to the floating pads of a water lily on an Ontario lake.

The outside of a leaf

• The blade is the flat part that catches light. The petiole is the stalk. Grass and corn leaves have no petiole: the base of the leaf wraps around the stem as a sheath.
• Simple leaves have one blade (maple, oak, birch). Compound leaves have a blade divided into separate leaflets (ash, sumac, clover). Tip: a bud sits at the base of a whole leaf, never at the base of a leaflet.
• Arrangement: opposite leaves come off the stem in pairs (maples, ashes, dogwoods); alternate leaves come off one at a time, staggered (oaks, birches, poplars).
• Venation: monocot leaves usually have parallel veins. Dicot leaves have net veins, either pinnate (one main midrib with side veins, like a feather) or palmate (several main veins spreading from one point, like fingers from a palm, as in a maple).

Inside a leaf: structure fits function

• Cuticle: waxy, waterproof coat over the epidermis. Thicker in dry or cold places.
• Upper and lower epidermis: one layer of clear, tightly fitted cells. Protects the leaf and lets light through. Except for guard cells, epidermal cells have few or no chloroplasts.
• Palisade mesophyll: one or more rows of tall cells packed with chloroplasts, right where the light is strongest. Chloroplasts can even move within the cell to catch more light or avoid too much.
• Spongy mesophyll: irregular cells and large air spaces that connect to the stomata, so CO₂ can diffuse to every cell and O₂ and water vapour can leave.
• Veins: vascular bundles. Xylem is usually on the upper side of a vein and phloem on the lower side. A bundle sheath of tightly packed cells surrounds each vein. Veins also act as a skeleton for the blade.
• Stomata: in most broad-leaved land plants, most stomata are on the lower surface, which is shaded and cooler, so less water is lost.

How guard cells work

Each stoma is surrounded by two guard cells. Their walls are thicker on the side facing the pore, and cellulose fibres wrap around them like hoops.

• Opening: in light, guard cells actively pump potassium ions (K⁺) in. The solute concentration rises, so water rushes in by osmosis. The guard cells swell, and because of their uneven walls they bow outward, opening the pore.
• Closing: in darkness, or when the plant is short of water, K⁺ leaves the guard cells, water follows by osmosis, the cells go limp, and the pore closes. A drought-stressed plant makes the hormone abscisic acid (ABA), which signals the guard cells to close (Lesson 12).

This is the plant's trade-off: open stomata mean more CO₂ and more photosynthesis, but also more water loss by transpiration.

Leaves adapted to their environment

• Conifer needles (spruce, pine, fir): small surface area, a thick cuticle, and sunken stomata in pits that trap still, moist air. These features reduce water loss in winter, when soil water is frozen and can't be absorbed. Most conifers keep their needles for several years, so they can photosynthesize on mild days in early spring.
• Floating leaves (water lilies): stomata on the upper surface, the only side touching air. The leaf and stalk contain aerenchyma, tissue with huge air spaces that keeps the pad afloat and carries oxygen down to roots buried in oxygen-poor mud. Cattails and rice also have aerenchyma.
• Sun leaves and shade leaves: on the same maple tree, leaves in full sun are thicker with more palisade layers; leaves in the shade are thinner and broader, catching more of the dim light.
• Modified leaves: cactus spines (reduce water loss and protect the plant), pea tendrils (for climbing), onion bulb scales (store food) and the leaves of carnivorous plants such as the pitcher plant, which trap insects to get nitrogen in poor, boggy soil.

Why leaves change colour in fall

Leaves contain green chlorophyll and yellow-orange carotenoids. In fall, shorter days and cool nights trigger the tree to break down chlorophyll and recycle its nutrients. The green fades and the yellows and oranges that were there all along show through. Some trees, such as red and sugar maples, also make red anthocyanin pigments in fall. Then a layer of cells at the base of the petiole (the abscission layer) seals off the leaf, and it falls.