Substrate

The substrate is everything filling the dish: its food, its walls, and the pH and temperature at every point in it. Growth media is the food, which most cells absorb straight through the membrane at a rate proportional to how much of it lies under them. Dense substrate is the wall: past a density threshold it stops a cell dead. Neither channel restocks itself, and every cell in the dish eats from the same stock.

Growth media

Membrane absorption is gated on one gene: a cell whose absorption yield is zero absorbs nothing from the field at all. Motile cells start at 1 and plasmodia at 40, on a range running to 80, and the value drifts multiplicatively under mutation. The same gene sets membrane permeability, so a cell that eats harder takes pH and heat damage faster through the same membrane.

The draw is proportional to the cell’s radius, to that yield, and to the media beneath it, so halving the media under a cell halves its intake. Below 0.01 of the channel it stops outright rather than tapering: a cell on nearly empty ground is not eating slowly, it is not eating.

Plasmodium branches and pseudopod segments read the field on their own rate, and for them the yield gene is a bonus rather than a gate, so a branching body feeds whether or not it carries it. Plasmodium branches stop at the same 0.01 cutoff. Transporters are separate income: they capture free nutrient particles and never read the field.

Everything absorbed is subtracted back out of the field on the next diffusion tick.

Dense substrate

Each frame a cell probes sixteen points around its own rim, at its actual membrane radius. Any probe over 0.34 latches the cell, pushes it out and cancels its velocity into the wall. The wall is drawn from 0.30, under the collision figure, so the tint arrives before the wall bites.

Opposing hits count as a squeeze. A squeezed cell, and a cell with most of its rim buried, accumulates pressure stress, and past a threshold that stress kills it. Driving into a wall fast enough kills outright instead. Both deaths skip an anchored cell, a cell mid-division and the cell the player is steering.

Dense ground is also the richest ground. Growth media is derived at generation from the final density, ramping from a basal floor on open ground up to full at the barrier density, so the best food in a dish sits under ground nothing can stand on. A dish generated at a lower terrain density is leaner everywhere, and a lane carved through rock keeps only the basal floor.

The in-game glossary entry mech.substrate tells the player that both forms carry nutrients cells can absorb. That is wrong for ordinary membrane absorption, which reads only the media channel. It is right for two morphologies, which that entry does not distinguish.

• A pseudopod digests whatever density its segments cross, at half the rate it takes media, with no gene and no threshold. Growing into bare rock still feeds the cell.
• A plasmodium can unlock barrier digestion. The gene ships locked at zero and opens on a two-percent roll at each mutation. Once open, branches digest density above the barrier and the mass is credited at five times its weight, against a maintenance cost per unit of branch length for carrying the gene at all.

Everything else has to break the wall down first, which takes a digestive enzyme or a lysocyst.

Replenishment

The dish does not refill by itself. The growth-media channel diffuses and does not decay, so it equalises gradients and never adds, and the dish edge is a zero-flux wall for it. A grazed patch refills from the media beside it; a field grazed everywhere refills from nothing.

Every unit that goes back in arrives as a ballistic particle depositing its energy as it decays: anything sprayed and not captured, mitochondrial waste, the byproduct a plasmodium’s branches shed, and the nutrient an enzyme mints out of the density it dissolves. Several particle types are excluded by name and never feed the field: growth media’s accelerant, the fragments a dying cell leaves, toxin, antibiotic, and the pH and thermal sprays.

Every strain in the dish, contaminants included, draws on the same field. A rival colony takes from the player’s intake wherever the two graze the same ground, not where the cells meet.

Starvation

1. Intake falls in proportion to the media beneath the cell, to zero at the cutoff.
2. Throughput falls with it. Mitochondrion conversion and nucleus fill are both multiplied by how full the cell’s own reserves are, measured against a baseline fill. The multiplier has a floor, so an empty cell is slowed to a fraction rather than stopped.
3. Division stalls. The nucleus fills at that scaled rate, so the first visible effect is a longer interval between divisions. It stops entirely only when the vacuoles are empty, because the transfer is clamped to what they hold.
4. Death comes last, and from cytosol rather than from the field. Cytosol is what runs out. Its starvation timer decays at the rate it wound rather than clearing, so a cell that starves and recovers repeatedly walks the clock forward instead of resetting it.

Clearing dense substrate

• A digestive enzyme dissolves it in pulses, each pulse bounded by the enzyme’s own consume strength and never digging below its dissolve floor. The energy released is bounded by the mass actually taken. An enzyme carrying the crystalline modifier deposits density instead.
• Pseudopod segments and barrier-digesting plasmodium branches write their consumption into the density channel, which is what leaves a hole behind them.
• Flow scours it where shear crosses a threshold and silts it back where the water slackens. Deposition can never raise a texel above 0.30, below the collision figure, so silt can shallow a lane but never close a passable one. This is off on an authored map, whose erodibility ships at zero and whose author has to opt in; a procedurally generated dish erodes.
• Four compound rows drive the density channel, all four toward zero.

A placed crystal does the opposite. Its body is dense substrate well above the density that stops a cell, so it is an obstacle as much as a chemistry source.

Growth media spray

Spraying growth media delivers nutrient particles, not a field write, so food reaches the field only where an uncaptured particle decays.

Alongside the food the spray emits accelerant particles, which carry no energy. Touching one stamps the cell for a decaying window, and three effects read that stamp: the vacuole-to-nucleus growth clock, a lowered divide bar with a bonus to replicate output, and intake itself, which scales the mitochondrion’s vacuole draw and membrane absorption. The stamp comes from contact rather than capture, so a cell with no transporter is boosted too, and the intake arm keys on a plasmodium’s parent cell, so its branches absorb faster while the stamp is fresh. Separately, the same spray triggers a growth boost on any network within reach of the brush.

The accelerant cannot create energy: every term it scales is a request against mass that already exists.