Desert geophytes under dew and fog: The “curly-whirlies” of Namaqualand (South Africa)
Abstract In the semidesert of Namaqualand and adjacent regions of the former Cape Province, South Africa, there occurs an assemblage of geophytes belonging to eight monocot families and some Oxalis species that exhibit special morphological adaptations of their aerial parts to harvest and absorb water from dew and fog, the main source of moisture in this region. Most of them pass their vegetative phase in winter. Leaves and in some cases axes, display a circinate, helical, tortuose, or serpentine shape and/or their margins are undulate or crispate, or (and) are provided with a ciliate or fimbriate pilosity of uncommon appearance. These morphomes, rare elsewhere among monocotyledons, promote an increased deposit of dew and fog by enlargement of surfaces and edges, keeping at the same time the overall size of the leaves restricted. They improve the water budget of these plants in three ways: (1) remnant water on the aerial parts retards the transpiration stress at day-time; (2) although special organs for direct absorption seem to be absent, field and laboratory tests show, that considerable uptake of water occurs but in quantities not exceeding that capacity found in many non-desert plants; (3) the water harvest of the leaves dripping to the soil and reaching the root zone, where it is stored in tubers, bulbs, corms and rhizomes, appears to be the main contribution. Experiments using artificial, directional fog and metal models imitating the natural profiles demonstrate that a surplus of water in efficiency rates of 0.1–66% is collected by the various surface types compared to a standard model with a non-sculptured (plain) frontal surface of the same size. The higher rates are sufficient to moisten the underlying soil down to the rhizosphere. The circumference of roots and storage organs probably does not exceed the area shadowed by the foliage. Namaqualand geophytes with such morphologies apparently form a novel desert biotype: as mesophytes they represent a distinguished strategy besides that of succulents and other xerophytes.