Rhizoids and Root Hairs of Ferns

Abstract

Probably a first adaptation of green plants to terrestrial life was the development of absorptive trichomes or rhizoids. The axis bearing these incorporated geotropic mechanisms and evolved a self-protecting pattern of mitosis at its apex, becoming a root. The recent work of Bierhorst (1968) and Roth (1963) in showing some relationships of Psilotum and Tmesipteris to the Filicales suggests the probability that all living vascular plants have descended from a single ancestor with roots, and that rootlessness in vascular plants is derived. Fern roots are usually very distinctive. The stele itself is nearly always colorless, but it is surrounded by one to several layers of heavy, brown, sclerified cortex. In many ferns the old cortex becomes a dense, almost black mass. In small roots there are usually only one or two sclerified cortical layers of cells surrounded by a layer of larger cells and usually a subepidermal layer with a more or less sclerified outer wall. In larger roots there are more cortical layers. The inner surface of the outermost cortical walls is likely to be studded with brown (phlobaphene?) tubercles; in extreme cases cortex, epidermis, and root hairs are full of such tubercles. Usually the impregnating material in the cortex is deposited in easily visible spirals with conspicuous pits between the gyres. In modern usage the term root-hairs usually refers to trichomes borne on roots, and rhizoids to absorptive trichomes borne on gametophytes or sometimes on sporophytic tissues. Both are believed to have the functions of absorption and anchorage. Traditionally, have been defined as trichomes borne on gametophytes and root hairs as trichomes borne on sporophytic roots or rhizomes (Haberlandt, 1909). However, Haberlandt did not know about the confusing similarities between the gametophytes and the subterranean axes of the sporophytes in the Psilotaceae, and in those days exaggerated importance was 107