In all except aerial roots and the non-anchored roots of aquatic plants, root hairs are usually present a short distance from the growing apex. These de-velop from the rhizodermis or root epidermis. Often the root hairs arise centrally from the basal part of the cell; occasionally they arise from near one end – this is a useful diagnostic feature. Again, whilst many root hair bases are level with other cells in the rhizodermis, in other plants they may be bulbous and protrude; they can be sunken into the outer cortical tissues (e.g. Stratiotes).
A short distance further away from the apex the root hairs often die and shrivel, but in some plants the root hairs persist for a long time. To the inner side of the rhizodermis an exodermis may develop, particularly in mono-cots. This characteristically consists of angular cells with somewhat thick-ened, lignified walls.
A multiple epidermis or velamen is found, for example, in the aerial roots of epiphytes (e.g. orchids; Fig. 4.1) and aroids. Frequently, cells in this situa-tion have specialized spiral or reticulate or irregular thickening bands and are capable of storing water absorbed from a humid atmosphere, mist or rain.