The exodermis of ash roots is initiated early in the apical meristem. When fully differentiated, it is composed of alternating “long” and “short” cells measuring approx. 70×25×25 μm and 25×28×25 μm respectively. At a short distance from the apex, the long cells undergo structural and histochemical changes from a “primary” towards a “secondary” stage: an impermeable suberized lamella is formed, cellulose lamellae become impregnated by lignin, and the protoplast dies off. The short cells show a distinctly thickened outer wall (“cap”) which is composed exclusively of cellulose, and possess abundant cytoplasm with a large nucleus. In the process of ageing the structure of short cells is not markedly affected. Following the early disintegration of rhizodermis, the exodermis works as the external protective layer of tho fragile end-root. Its long cells fulfil the mechanical protection, the short cells serve as passage cells for solutions and gases, during the whole life-span of the rootlet.