In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem.
The epithem is penetrated by a network of fine intercellular spaces, which are normally filled with water and debouch on one or more intercellular cavities below the epidermis.
The epithem is frequently surrounded by a sheath of cuticularized cells.
In other cases the epithem may be absent altogether, the tracheal strand debouching directly on the lacunae of the mesophyll.
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