St. Porembski’s research while affiliated with University of Bonn and other places

Plants of the monotypic Blossfeldia liliputana have the smallest bodies of all Cactaceae. The button-like plants with a diameter of usually some 10 mm occur in rock crevices in arid regions between S Bolivia and N Argentina. Based on observations and experiments in the field and in cultivation, morphology, anatomy, reproductive biology, certain aspects of ecophysiology, and behaviour under water stress are described. The very small flowers are autogamous; the arillate hairy seeds are unique within the family and represent a particular adaptation to ant dispersal. These CAM-plants virtually lack stomata: 0.6 stomata/mm2 represents the lowest number in terrestrial autotrophic vascular plants. However, all other xeromorphic features characteristic for globular cacti are absent (e.g. no thickened cuticle, no thickended outer cell walls, no thickened hypodermal layers). These features allow a high degree of desiccation: under water stress the plants lose up to 80% of their weight within one year and can withstand an additional drought of at least another year. Thus Blossfeldia is poikilohydric like many lichens and mosses and represents the unique life form of a succulent resurrection plant.

:The dwarf globular cactus Aztekium ritteri shows a high degree of surface differentiation. The stem consists of 8–15 ribs, these in turn being sculptured by transverse furrows: compared with a sphere of equal size, the surface/volume ratio is increased by more than 10-fold. At the same time, only 20% of the stem surface is exposed to direct sunlight, 80% lying in the shade of its own body, in a system of furrows and crypts. The exposed areas have few stomata and the epidermal cell walls are notably thickened and cutinised. In addition, these form an exoskeleton by means of a hypodermis containing calcium oxalate crystals while the unexposed cell surface regions, hidden in the furrows, lack xeromorphic characters and calcium oxalate crystals. By far the greatest number of stomata are found in the furrows mostly in deep crypts. When dehydration occurs as a consequence of longer periods of drought, the furrows and crypts close together, thereby reducing transpiration losses. The complicated interlocking nature of the surface architecture of Aztekium is discussed in terms of the functional aspects as an adaptation to an extreme habitat (vertical shale cliffs in Nuevo León, Mexico).