Comparative Anatomy of the Stem in the Carrion Flowers (Apocynaceae-Asclepiadoideae)
The carrion flowers or stapeliads (Stapelia and related genera, Apocynaceae) comprise a group of several hundred species of succulent plants native to the Old World, primarily Africa. Despite the size and diversity of the group, surprisingly little is known of the comparative anatomy and morphology of its members. This study documents stem anatomical structure and cuticular characteristics of twenty-eight species representing fourteen genera of stapeliads. Materials were prepared for examination using standard histological techniques. The principal anatomical features examined in stem cross sections were pith radius and cortex thickness and the presence/absence of a thickened outer epidermal wall, hypodermis, palisade cortex, cortical bundles, collapsible cortex, and medullary bundles. Cuticular features examined included epidermal cell wall characteristics, subsidiary cell arrangement, presence/absence of trichomes, papillae, and cuticular striations, and calculation of the stomata! index. Observations showed most species to have distinctive adaptations to the stem succulent habit, including the loss of leaves, development of ribbed stems, production of abundant water storage tissues development of a perennial epidermis, occurrence of stomata in the stem epidermis, and development of columnar cortical cells with numerous chloroplasts and large intercellular spaces. The cortex/pith ratio ranged from 0.4 to 1. 7 in all but one species (Pseudolithos eylensis), indicating that the cortex contributes a larger storage volume than the pith. The outer epidermal cell walls were thickened in all species. Hypodermis was absent from most species, but present in species of Duvalia and in Stapelia engleriana. A palisade cortex was present in more than half of the species. Cortical bundles were generally absent, except in the genus Echidnopsis. A collapsible cortex was absent in most species, and medullary bundles were seen only in Cara/Zuma diffusa and Duvaliandra dioscoridis. Epidermal cells were usually hexagonal or pentagonal in shape with straight end walls and were generally not aligned in rows. Subsidiary cell arrangement was varied, with cyclocytic, brachyparacytic, and anomocytic arrangements being most common. The stomata! index ranged from 0.9 to 5.4. Most of these features are common to stem succulent plants in general, although some features characteristic of other stem succulents, most notably the cacti, are generally absent from the stapeliads.
apocynaceae, Asclepiadaceae, stems, botany, morphology, anatomy
Kajoina, F. (2010). Comparative anatomy of the stem in the carrion flowers (Apocynaceae-Asclepiadoideae) (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.