In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of a single layer of cells has typically thickened and cuticularized outer walls.
The epithem is frequently surrounded by a sheath of cuticularized cells.
There is no need for cuticularization here, as the external dangerous influences do not reach the interior, and the processes of absorption which Boussingault attributed to the external cuticularized cells can take place freely through the, delicate cell-walls of the interior, saturated as these are with water.
They are without stomata on their submerged portions, and the entry of gases can only take place by diffusion from the water through their external cells, which are not cuticularized.
The amount of watery vapour in the air passing through a stoma has no effect upon it, as the surfaces of the guard cells abutting on the air chamber are strongly cuticularized, and therefore impermeable.