Welwitschiales

Abstract

The adult sporophyte of Welwitschia has no parallel in the plant kingdom. It resembles a gigantic turnip, and reaches a diameter of more than 1 m. The stem consists of a tough axis shaped like an inverted cone, and has a deep apical depression. Most of the stem is buried in the sandy soil, its lower part tapering abruptly to an extremely long carrot-like tap root which penetrates the soil between 1.0 and 1.5 m deep before it splits into numerous thin roots. How far downward the roots extend is still uncertain. The exposed portion of the stem consists of a massive, woody, concave disc which bears two opposite and decussate strap-shaped leaves (Fig. 19.1 A, B). These leaves grow continuously throughout the life of the plant but very slowly, ca. 10–15 cm in a year (Von Willart 1985), from a basal meristem, and the tip dies back continuously. A mature leaf is ca. 3 m long and 1–1.5 mm thick. The distal end of the leaves splits and becomes frayed and extends in a twisted or contorted manner along the surface of ground. Old plants may be more than 1000 years (Bierhorst 1971). A specimen ca. 2000 years old is shown in Fig. 19.1 A.