Development
There are three major epidermal cell types which all ultimately derive from the L1 tissue layer of the shoot apical meristem, called protodermal cells: trichomes, pavement cells and guard cells, all of which are arranged in a non-random, non-predictable fashion.
Production is reliant on interactions between SPCH (speechless), EPF (downregulates stomata), TMM (too many mouths, downregulates stomata) and stomagen (upregulates stomata, inhibits SPCH), ERL and YODA downregulate stomata too.
Stomata positioning is down to CO2 activating EPF1, which activates TMM/ERL which together activate YODA, YODA in turn inhibits SPCH, in turn SPCH activation decreases, allowing asymmetry.
An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround a stoma. This meristemoid then divides asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cells.
Read more about this topic: Stoma
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