Stoma

Every leaf is covered in tiny mouths. A stoma is a pore in the epidermis, flanked by two guard cells, and it is the one door a leaf has for trading CO2 for water vapor with the outside air — open it too little and the plant starves for carbon, open it too much and the plant dries out. Nothing about the pore is fixed: the guard cells around it are alive, and they spend the whole day adjusting how far apart they sit.

What actually moves them is two separate, separately measured facts stitched into one causal chain. Kinoshita & Shimazaki worked out the trigger in 1999: blue light hits a phototropin receptor in the guard cell's own membrane, which activates a proton pump in that membrane (the plasma-membrane H+-ATPase), which drives H+ out and K+ in — and the guard cells swell as water follows the salt, the same osmotic swelling that plumps any cell handed more solute to hold onto. What that swelling does to the pore itself, Franks, Cowan, Tyerman, Cleary, Lloyd & Farquhar measured directly in 1995, bypassing light and chemistry altogether: they filled real guard cells with a pressure probe, held them at chosen turgor pressures from 0.0 to 4.1 MPa, and read the pore's width off at each one. The relationship was sigmoidal — barely open at low pressure, opening fast through the middle of the range, flattening toward a near-maximum aperture right around 4.1 MPa.

Drag the slider and the pore's real µm reading grows alongside it. Outlaw & De Vlieghere-He measured actual Vicia faba pores across one real morning in 2001, from 2.0 ± 1.6 µm near dawn to 7.4 ± 2.3 µm by late morning — this room's own scale tops out at that 7.4 µm figure, so a fully drawn pore here is reporting a number a real microscope has actually seen on this exact species, not a round number invented for the slider. The pore itself is drawn dark on purpose in both light and dark mode — it's a hole into the leaf's own interior, which stays dark regardless of what hour you're reading this at, the same fixed-dark choice /pulse's oscilloscope already makes for a different reason.

Honest gap: no single paper measured this whole chain end to end, and this room doesn't pretend one did. Franks et al.'s pressure probe set turgor directly, with no light and no H+-ATPase anywhere in the apparatus — real blue light never produces a chosen MPa value on command, so the slider here is standing in for the probe, not for the sun. The aperture curve itself is a plain logistic, calibrated to the two things the paper states in words (closed at zero pressure, sigmoidal, near-maximum at 4.1 MPa) rather than digitized from their own figure, which this room doesn't have the underlying data points for. The 7.4 µm ceiling comes from a third, unrelated morning's measurements on real leaves in real ambient light, stitched on as a scale reference for the pressure curve's own output, not a claim that 4.1 MPa specifically produces 7.4 µm in any one plant. And the response here is instant; a real guard cell's H+-ATPase phosphorylates within tens of seconds of a light pulse, but the water flux that follows and actually swells the cell runs on the order of minutes — the same speed-up liberty /cone and /thaw already take with their own real, much slower mechanisms.

Sources: Kinoshita & Shimazaki, Blue Light Activates the Plasma Membrane H+-ATPase by Phosphorylation of the C-Terminus in Stomatal Guard Cells, The EMBO Journal 18(20), 5548–5558 (1999); Franks, Cowan, Tyerman, Cleary, Lloyd & Farquhar, Guard Cell Pressure/Aperture Characteristics Measured with the Pressure Probe, Plant, Cell & Environment 18(7), 795–800 (1995); Outlaw & De Vlieghere-He, Transpiration Rate: An Important Factor Controlling the Sucrose Content of the Guard Cell Apoplast of Broad Bean, Plant Physiology 126(4), 1716–1724 (2001).