FUNCTIONAL-MORPHOLOGY OF AZTEKIUM-RITTERI (CACTACEAE)

The dwarf globular cactus Aztekium ritteri shows a high degree of surf ace differentiation. The stem consists of 8-15 ribs, these in turn bei ng sculptured by transverse furrows: compared with a sphere of equal s ize, the surface/volume ratio is increased by more than 10-fold. At th e same time, only 20 % of the stem surface is exposed to direct sunlig ht, 80 % lying in the shade of its own body, in a system of furrows an d crypts. The exposed areas have few stomata and the epidermal cell wa lls are notably thickened and cutinised. In addition, these form an ex oskeleton 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 great est 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 architectu re of Aztekium is discussed in terms of the functional aspects as an a daptation to an extreme habitat (vertical share cliffs in Nuevo Leon, Mexico).