Inside a leaf
A leaf is where photosynthesis happens, and the pores that let carbon dioxide in also let water vapor out, so a leaf is always trading carbon gain against water loss (Roth-Nebelsick & Krause, 2023). The same evaporation is also how it cools itself (Michaels et al., 2022).
The layers
Top to bottom: a waxy cuticle (waterproofing), an upper epidermis (the leaf's outer skin of cells), the palisade mesophyll , the spongy mesophyll, the vascular bundles (veins, bringing water in and sugar out), and a lower epidermis, where the stomata sit (Michaels et al., 2022; Clark et al., 2018).
The palisade layer is tightly packed and dense with chloroplasts, the green compartments inside a cell where photosynthesis happens. The spongy layer below it is loose, with air spaces for gas to circulate.
Fig. 7. The palisade layer is built to catch light; the air spaces of the spongy layer let gases move; the veins bring water in and carry sugar out; the stomata on the underside handle gas exchange.
Stomata : the adjustable pores
In many flowering plants, a stoma is a tiny pore between two bean-shaped guard cells; grasses use a dumbbell-shaped pair instead. When the guard cells fill with water, the pressure inside them makes them curve apart and open the pore between them; when they lose water they straighten and the pore closes. Essentially hydraulic. Exactly how the walls manage it is still debated (Roth-Nebelsick & Krause, 2023), so don't let anyone tell you it's settled.
Light opens them, to take in CO2. The strongest signal for closing them is water stress: once a leaf is short of water, the stomata close whatever else is going on (Roth-Nebelsick & Krause, 2023). In flowering plants, dry air prompts the leaf to make a hormone, abscisic acid , within minutes, and the hormone sets ions flowing out of the guard cells so they go slack and the pore closes (Sussmilch & McAdam, 2017). Dry substrate is thought to get there by a longer route: the roots sense the water shortage first and send signals up to the leaves, where most of that hormone is made (Takahashi et al., 2020). In one study, warmth worked both ways. Tropical tree saplings had their leaves warmed from about 90 to 108 °F (32 to 43 °C). While the air's drying pull on the leaf (what scientists call vapor pressure deficit) was held steady, the stomata did not close right away and at first opened wider; when that drying pull was allowed to climb with the temperature, they narrowed steadily (Cheesman et al., 2026). A heated room in winter is the drying case: heating raises how much moisture the air can hold, and there is rarely enough moisture around to fill it, so the air gets drier (University of New Hampshire Extension, 2025). We haven't found that measured on houseplants, and a living room is far cooler than those leaves were; that houseplants respond the same way is our reasoning from the two. As they narrow, they hold back photosynthesis (Cheesman et al., 2026). That's why we'd expect a plant in a sunny, very dry room to be flooded with light and still barely growing, and why we'd reach for humidity and water before shade. That's our reasoning too, not a measured result.
Fig. 8. Guard cells open (left) when full of water, closed (right) when not. The plant trades water loss for CO2 every time its stomata open (Michaels et al., 2022).
Strengthen your understanding
Try these in your head first, then open to see.