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Stomata are often absent, absorption and excretion of gases in solution being carried on through the epidermal layer.
The epidermal cells may contain chlorophyll.
In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex.
Mention may be made here of a class of epidermal organ, the hydaihodea, the wide distribution and variety of which have been revealed by recent research.
In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connection with the vascular system.