Plants grow, compete for room, evolve, and die in a flat arena. There is no landscape here and no weather — the ground is the same everywhere, and a spot is either free or it is not. The only thing a plant is up against is the other plants.

That is the whole idea. Nothing is adapted to a climate, because there is none. Every node lives the same fixed span, and what decides whether a lineage persists is the shape it grows: how many branches it puts out, how far apart, at what angles, how quickly, and how all of that changes along the plant as it ages. Room is the only resource, and the forms that claim it and hold it are the ones that go on.

Every plant has a leaf. Lance, spade, heart, clover, needles, fern, aspen, wheat, dandelion, oak, maple or horse chestnut: a plant keeps one shape for life, and so do the pieces that break off it. A new plant takes a shape none of its neighbors has, where there is one to take.

Color is ancestry. Every new plant is born with a hue of its own, chosen to stand apart from the plants already there, and passes it down. Each time a plant branches the hue may shift a little, so a lineage's color drifts as it evolves and a piece that breaks off keeps the color of the plant it came from. How vivid a node is says how sharply it turned from its parent — a straight vine is grey, a wide fan is vivid. How light it is says how old it is: new growth is pale and darkens over its life, so a plant is bright at its growing edge and deep in its old interior.

What a plant inherits is a short program, not a set of numbers. Every node runs it to decide how many children it may have, how far away to put them, which way to turn, how fast to grow up, and how much it wants to grow right now. Because the program can read where the node finds itself — its age, its depth in the plant, where it stands, how crowded it is — one plant can carry both kinds of node at once: nurseries that sit and reproduce, scouts that run. If you cannot see both inside a single plant, the system is not working, whatever the diversity numbers say.

Plants come apart. A node that dies while still holding children does not vanish: it stops growing and stays as structure, which is what heartwood is in a real trunk too. Eventually it rots through, and whatever was hanging off it becomes a plant in its own right. That is how new individuals mostly arrive here — not from seed, but by bodies breaking apart where they already stand.

Plants have parents. Every plant is scored on the ground it has covered, averaged over its life, and on the enemy nodes it has killed. When a plant that was doing well dies, a new one grows in the middle of the largest open space in the arena. One time in four it is a stranger with a random program. Otherwise it is the child of two of the five best-scoring plants, chosen to be as unlike each other as possible: the stronger gives its program and the weaker overwrites a stretch of it, as much as half when the two are evenly matched. The child's color and leaf are its own, chosen to stand apart from the plants already there. And when the arena holds too many plants, the one cleared is the oldest of those covering the least ground.

Plants move. A plant's root is fixed, but every other node can turn the branch that joins it to its parent, and the turn carries everything beyond it. A joint slows under what it carries, so the tips keep moving at any age, the stems settle, and a long limb is swung slowly whatever it weighs. A branch swings only so far either side of the angle it grew at, slowing as it nears each limit and turning back; it bends, it does not spin. The program decides how fast and which way, and it can feel where the nearest other plant is. When two plants touch, the faster of the two nodes wins and the slower is cut, losing everything beyond the point of contact, and the branch that struck lives longer for every node it took. The core of a plant, the first ten generations out from its root, is hard: it cuts whatever strikes it. So is any branch heavy enough, which is what makes a trunk a trunk. A plant may pass through itself. So reach is a weapon and a liability at once, speed is what decides a fight, and what a lineage does with that is up to it.

The arena is 16 by 9 and it has edges. Whatever shape your window is, the world is the same world; the black bars are outside it. A branch may be carried past the frame, but nothing grows there: a node outside the arena, or a child that would land outside it, is refused. Nothing wraps around to the other side.

The graphs show how the meadow has changed since you arrived: how many plants and how large, how much of their programs they use, whether they remember anything, and how fast they move.

Watching

• Press a plant to highlight the whole of it and count its nodes. Drag across the arena to sweep from one to the next.
• A plant is often larger than it looks: two or three patches of living growth strung together on a thread of dead wood are one organism, and the highlight takes all of it.

Controls

The ground

The photograph under the plants is of moss and soil, by Liam Briese, from Unsplash. The settings page can dim it, or turn it off.

Sharing

The settings page has a link you can copy to share your world and its settings. A link always starts its world from the beginning, so leave it running to reach what you were looking at. And if you changed settings part way through, it will not even do that — what you were watching took a path that a list of settings cannot describe. The panel tells you which of the two you have, underneath the link.