Surface Structure

Whether covered with leaf armour or not, the surface of the fern stem is protected by an epidermis, or “skin,” a single layer of epidermal cells, which are more or less flat cells with thick outer walls. Most fern stems also are covered with a protective indument, consisting of hairs, known as trichomes, or scales; these are so distinctive that they are valuable in identification and classification. The indument includes such diverse types of epidermal emergences as simple glands (unbranched one- to several-celled trichomes with a headlike cluster of secretory terminal cells), simple (unbranched) nonglandular trichomes, dendroid trichomes (branching filaments), and scales (flat cell plates) of many patterns. Scales (also known as paleae) are each defined as a cell plate two or more cell rows wide, at least at the base, whereas hairs each generally consist of a single row of cells. Transitional states are also known.