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Editor: Wilhelm Giess
- Parmelia hueana Gyeln., a vagrant lichen from the Namib Desert, SWA/Namibia. I Anatomical and reproductive adaptations. B. Budel and D.C.J. Wessels
- Drought resistance in water plants in rock pools of Southern Africa. D.F. Gaff and W. Giess
D.F. Gaff and W. Giess
ABSTRACT: Water plants growing in ephemeral pools on rock outcrops in arid areas of southern Africa exemplify contrasting adaptation to drought. Some possess leaves or rhizomes that can tolerate dehydration. Aponogeton desertorum dies back to perennating organs which resist excessive water loss. The former group experience extreme fluctuations of temperature and aridity; they have evolved tolerance of complete desiccation.
INTRODUCTION: Early botanists classified plants according to habitats into xerophytes, mesophytes and hydrophytes (adapted to arid, well-watered, or aquatic habitats respectively). Xerophytes frequently possess anatomical features which were thought to restrict water loss. The difficulties of such classifications are exemplified by the water-plants which grow in hollows in granitic rock outcrops (Giess 1969, 1970; Mauve 1966) in the arid regions of southern Africa adjoining the Atlantic Ocean. These grow as typical hydrophytes with aerenchymatous tissue in floating or submerged leaves when the rock-hollows are temporarily filled with rain water, but they are extremely drought resistant when the pools are dry (Heil 1925; Hickel 1967).
Drought resistance has been subcategorized into the ability of tissue to recover from reduced water content (i.e. drought tolerance) and the ability to minimize reduction in water content when external conditions are adverse (i.e. drought avoidance; see Levitt 1980). This paper examines the protoplasmic drought tolerance of the drought resistant water plants of South Africa, as part of a search for 'resurrection plants, i.e. plants whose protoplasm survives air-dryness. As this involved continual travel, ecological data were of necessity, restricted.
METHODS: Plants were collected by the authors (in August and September) during the dry season in southern Africa. With the exception of rhizomes of Aponogeton desertorum, all plants were dry, shrivelled and discoloured either brown or purple-black. [...]