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A leaf is a stack of thin, living layers. Light comes in through a clear, waxy skin to the tall palisade cells packed with chloroplasts, and air moves in and out through stomata, the tiny pores between pairs of guard cells on the underside.

Guard cells open a stoma by taking in potassium. Water follows the potassium in, the cells swell and bow apart, and the pore opens: carbon dioxide drifts in to be built into sugar, while water vapor drifts out and draws a fresh column of water up from the roots.

Sunlight passes the clear skin into the palisade cells. Their chloroplasts soak up red and blue light near the surface, while more of the green light travels deeper and is used by the cells farther down.

A spray droplet lands on the cuticle, the waxy coat that keeps a leaf from drying out. Whatever a leaf takes in from a foliar spray has to cross that coat or move in along the walls of an open stoma.

Calcium travels up from the roots in the water stream and goes wherever that water is drawn, mostly to leaves losing vapor through their pores. There it settles into the cell walls and helps hold them together. Once in place it barely moves again, so new growth depends on a steady supply arriving with the water.

Two sets of pipes run the length of every stem. Xylem carries water and dissolved minerals up from the roots, and phloem carries the sugars the leaves make down to the roots and out to the fruit and the growing tips.

Water climbs the xylem in an unbroken column, pulled from above as the leaves lose vapor. Its conducting cells are hollow and no longer alive, stiffened by coiled walls, and pits in those walls let water pass sideways from one to the next.

Phloem moves sugar from the leaves that make it to the parts that spend it: the roots, the flowers, the fruit and every new shoot. The sap flows through living sieve tubes, cell to cell, across the sieve plates that join them end to end.

In many plants sugar is first pushed out of the leaf cells that hold it, then pulled into the companion cells and sieve tubes by sugar pumps. Potassium helps keep those cells charged so the pumps can keep working.

When a leaf is bitten or chilled, an electrical pulse races away from the spot along the living sieve tubes, much like a slow nerve. It carries the signal to leaves farther off, which start making their defenses before any harm reaches them. The potassium packed inside the tubes is part of what lets them carry the charge.

Thousands of root hairs, each one a single cell stretched into a thread, multiply the surface that touches the soil. Water and dissolved minerals cross into the hairs, move inward through the root, and enter the xylem for the trip up the stem.

Water crosses into a root hair and moves inward through the cortex, both through the living cells and along the walls between them. The pull comes from above: as the leaves lose water vapor, the xylem draws more in behind it.

Water can travel through the porous cell walls or pass from cell to cell through tiny channels. At the endodermis a waterproof band, the Casparian strip, seals the walls, so everything has to cross a living cell membrane, which lets the root control what enters the xylem.

The cap shields the tip as it pushes through the soil and sheds border cells into a coat of mucilage, a gel that keeps them alive while they sense the soil and come into contact with its microbes. Behind the cap, cells divide and then lengthen, and that lengthening drives the root forward.

The outer cells of the root spend energy pumping protons out into the soil water. That builds a charge across each cell's membrane, and the charge is what pulls potassium and many other nutrients in. As it works, the root can also shift how acid the soil right around it is, depending on what it is taking up.

A single leaf cell can hold dozens of chloroplasts. Inside each one, stacks of green membranes called grana catch light, and the fluid around them, the stroma, uses that energy to build sugar from carbon dioxide and water.

Light striking the stacked membranes splits water, releasing oxygen and packing hydrogen ions inside each disc. As those ions flow back out through ATP synthase, an enzyme set in the membrane, their flow makes ATP, the energy the chloroplast builds with.

Carbon dioxide drifts into the stroma, where an enzyme called Rubisco fixes it into sugar. Rubisco is so slow that it makes up a large share of all the protein in a leaf. The sugar either moves out to the rest of the cell or is stored in the chloroplast as starch.

Magnesium sits at the center of every chlorophyll molecule, so the green that catches light is partly built from it. In the stroma around the grana it also helps switch Rubisco on, and the ATP the chloroplast makes does its work bound to magnesium. So a leaf short of magnesium catches less light and fixes less carbon.

The narrow band of soil around a root teems with far more life than the soil a short way off. Roots feed it with sugars and other compounds, and in return fungi and bacteria help bring in nutrients the roots could not reach alone.

Roots release sugars, organic acids, amino acids and mucilage into the soil around them. Beneficial bacteria sense these exudates, swim toward the root and settle on it, and many of them help the plant grow.

Phosphate barely moves in soil, and much of it is held on grains coated with iron, aluminum or calcium. Organic acids from the root, such as citrate and malate, release it by taking its place on the grain or by binding the metal that held it, and the root takes it in.

Mycorrhizal fungi grow into the root and branch into treelike arbuscules inside its cortex cells. The plant supplies them with carbon, as sugars and fats, and their threads, finer than any root hair, reach far beyond the hairs to gather phosphate and carry it back.

Many of the bacteria living around a root make the same growth hormones the plant uses itself. When the root takes them up, it can grow longer, sprout more root hairs and push out new side roots. More root means more soil within reach, and more surface for helpful microbes to settle on.

Many aromatic herbs, lavender among them, are dotted with glandular trichomes. Each one is a short stalk topped with a disc of secretory cells that make terpenes, the oils behind a plant's scent, and store them under a dome of cuticle.

Sugar made in the leaf travels up through the stalk into the secretory cells, where it supplies both the energy and the raw material for making terpenes.

Plastids in the secretory cells build terpenes from that sugar. The oil passes out through each cell's membrane and wall into the space under the cuticle, which swells into a dome as it fills, keeping the oil away from the living cells it would harm.

Because the oil is stored ready-made, a plant can respond to an attack at once. Once released, its volatile terpenes deter many insects, help protect against microbes and can draw in pollinators.

Each gland starts as a single cell on the leaf's surface. It divides into a basal cell, a short stalk and a disc of secretory cells, and the cuticle on top lifts away to leave an empty dome ready for oil. Calcium gathers in the young gland's walls and under that lifting cuticle, and it seems to help the gland form properly.