One cell layer that seals a plant from the world
A plant’s epidermis—Greek for “over-skin”—is usually a single cell layer coating leaves, flowers, roots, and stems. It blocks water loss, manages gas exchange, and hosts guard cells and trichomes. Modern botany classes it as dermal tissue, not mere specialised parenchyma.
Most species keep the sheet one cell thick; rubber fig and Peperomia can stack multiple layers after periclinal divisions in the leaf protoderm. Epidermal cells are the commonest and least specialised, typically more elongated on monocot leaves than on dicot ones. Beside them sit guard cells, subsidiaries, and hairs.
Trichomes spike from many shoots; on roots, hair-like outgrowths specialise in drinking water and minerals. That division of labour turns a boundary layer into both armour and absorbent brush depending on organ.
In roots and stems that thicken with age, a cork cambium eventually swaps the epidermis for a periderm. Leaves and stems are dotted with stomata, each pore flanked by two bean-shaped guard cells and two to four subsidiary cells. Guard cells are the only epidermal cells with chloroplasts, and so the only ones that make sugar. On one leading theory, sunlight raises their potassium and sugar content, water rushes in by osmosis, they swell, and the pore opens; at night the sugar runs out, water leaves, and the pore shuts to save moisture. Hair formation in the model plant Arabidopsis thaliana depends on the genes TTG and GL1, with gibberellins also involved, while inhibitors such as TRY spread to neighbouring cells and keep them flat. Later expression of the transcription factor MIXTA turns petal cells conical. Stomata are harder to study, since mutants often die, but their density responds to surroundings, and hormones such as ethylene appear to raise it when plants are kept enclosed. In dicots stomatal spacing looks almost random, whereas monocots place them through precise asymmetric divisions.
Source: Epidermis (botany)
