A Survey of Seed Surface Morphology in Hesperantha (Iridaceae)

Abstract

Seeds of a range of species of Hesperantha were examined with light and scanning electron microscope (SEM) and compared with three species of the most closely related genus, Geissorhiza. A turbinate to globose shape with a persistent funicle and a testa of uncontorted epidermal cells with smooth surfaces is apparently the basic seed type. Modifications of the basic type include increasing compression and a triangular or more or less irregular shape, sometimes accompanied by wrinkling and crumpling of the epidermal cells, and the development of dual tails and wings. This is most marked within Hesperantha sect. Radiata (but not H. marlothii of the section), where these modifications are accompanied by reduction in seed size. Wing-like structures are also developed within sections Hesperantha (H. spicata and H. cedarmontana) and Concentrica (H. fibrosa) but in both of these groups without accompanying crumpling of the epidermal cells, which coincidentally have developed rough surfaces. Hesperantha Ker is a genus of some 55 species of corm bearing perennials of Iridaceae subfamily Ixioideae. It is centered in Southern Africa, but a few species occur in the montane areas of tropical Africa, extending as far north as Cameroon and Ethiopia. The genus has recently been revised for the winter rainfall area of South Africa by Goldblatt (1982, 1984) and is being studied in eastern Southern Africa by Hilliard and Burtt (1979, 1982). This study of seed morphology in Hesperantha was made in conjunction with the revisionary work, now completed or in progress. Seed morphology of Iridaceae is in general poorly known and thus seldom has been of taxonomic value below the generic level. Differences in seeds between genera are, however, sometimes striking and may provide important generic characteristics. Good examples are the circumferentially winged seeds of Gladiolus and its close allies, the two winged seeds of Watsonia, and the inflated seeds with spongy testa of Tritoniopsis and Anapalina. This study was thus undertaken in the hope that some characteristics of taxonomic use would be found in Hesperantha at species and generic level. Seventeen species of Hesperantha, including examples from all four sections (Goldblatt, 1982), as well as three species of the related genus Geissorhiza Ker were assembled for light and scanning electron microscope examination. This represents a large sample for a monocot genus such as Hesperantha, in which seeds are typically produced after flowering and are seldom collected and consequently poorly known. Seeds of Hesperantha have not previously been studied in detail, but SEM studies of seed morphology have been made in a few other genera of subfamily Ixioideae, in conjunction with the systematics notably in Syringodea (de Vos, 1974) and in Crocus (Baytop et al., 1975; Mathew, 1976). In Crocus some interesting seed surface features including trichomes and papillae have been found to be of taxonomic significance. In ystematic studies of other genera of Ixioideae, seed morphology is occasionally of limited taxonomic use, as in Tritonia (de Vos, 1982: 113) where one or two species stand out from their allies in having unusual seed modifications. The extensive study of Huber (1969) on the seed morphology of the monocotyledons deals largely with internal seed structure, and not at all with detailed surface microstructure such as is observed with the SEM. His observations on Hesperantha are very general and relate primarily to tribal and familial classification. 1 This research was supported by Grant DEB 78-10655 and DEB 81-19292 from the United States National Science Foundation. We thank Mike Veith, Washington University, St. Louis, for his assistance in the SEM work. 2Bishop Museum, P.O. Box 19000-A, Honolulu, Hawai'i 96817. 3B. A. Krukoff Curator of African Botany, Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166. ANN. MISSOURI BOT. GARD. 71: 181-190. 1984. This content downloaded from 157.55.39.49 on Tue, 09 Aug 2016 05:26:20 UTC All use subject to http://about.jstor.org/terms 182 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 71 MATERIALS AND METHODS The seeds of 16 species of Hesperantha (about a third of the genus) were examined by scanning electron microscopy (SEM). Sufficient viable seeds were studied to document the variation present within each population. Four to ten seeds were generally adequate. Three additional species of the closely related genus Geissorhiza were also examined as out-groups in the hope that this would aid in the establishment of character polarity (Table 1). One population only of all species except H. marlothii was examined with the SEM. The variation in seed size in H. marlothii was such that two populations were studied, covering the extremes encountered in the species. Samples of several populations of H. falcata and H. radiata were also examined under the light microscope to determine whether material studied was typical of the species. In all cases they matched closely the samples studied. One more species, H. pauciflora (sect. Hesperantha) was also examined under the light microscope and since it had seeds exactly like those of H. falcata (also sect. Hesperantha), it was not examined further. The number of populations examined may appear to be too small to gauge the variation within species but as indicated in the introduction, seed samples are difficult to obtain in Hesperantha in which plants are seldom collected in fruit. The material studied here therefore represents an unusually large assemblage. Where more than one population was available, as in H. falcata and H. radiata, the seed was examined with the light microscope and found to match the seed of the population studied with the SEM. Thus as far as it is possible to estimate, the single populations studied appear to be representative ofthe species. Several of the species examined (H. elsiae, H. purpurea, H. brevifolia, H. cedarmontana) are known from one or very few populations and are so restricted in their distributions that the material studied here represents a good sampling of the species. Among the widespread species, several populations were checked in H. falcata and H. radiata while only single samples were available in H. bachmannii and H. pilosa. Viable seeds were attached with water soluble white glue to aluminum stubs, coated with 500700 A of gold in a sputter coater, and examined in a Hitachi S-450 SEM at 15 kV and 60-80 jA. Photomicrographs were made with Type 5 5 P/N Polaroid film. Contact prints were made on Illford No. 2 paper. Surface features are presented at two magnitudes of magnification: 35 x-90 x to show overall seed shape and surface topography, and 1,000 x to resolve the microsculpture of the epidermal cell surface. DESCRIPTION OF SEED FEATURES The variation in the seeds of Hesperantha species is often limited to relatively minor modifications that produce large differences in seed shape, sometimes even within one capsule. There are, however, certain patterns of variation that appear, at least from the small samples available, to be characteristic of particular species. The variation within Hesperantha as well as that found in a sample of three species of the closely related genus Geissorhiza is presented in Figures 1-29. Only 13 of the 16 species of Hesperantha examined are illustrated. The additional species studied add no significant information to the observed pattern of variation. The variation is described in the following pages in sections dealing with shape, size, color, surface morphology, and microsculpturing. Shape. The basic shape of Hesperantha seeds is turbinate to turbinate-globose. The embryo containing portion is globose to ovoid and this is modified to a turbinate shape by the persistent funiculus present on the seeds of many of the species (e.g., Figs. 1, 3, 5, 8). The same basic shape is evident in Geissorhiza (Figs. 16, 17). The more regular globose shape, illustrated here by H. erecta (sect. Concentrica) and H. falcata and H. luticola (both sect. Hesperantha) (Figs. 1, 5, 8), occurs in species of all sections of Hesperantha except sect. Radiata, as well as in Geissorhiza (G. humilis). The seed is more abruptly constricted to the persistent funiculus in these species. The basic turbinate to globose shape is modified in a number of the species. The modifications can be grouped into three different types. An irregularly wrinkled surface occurs in several species of Hesperantha distributed in several sections (e.g., H. bachmannii, Fig. 4; H. muirli, Fig. 13) and in Geissorhiza burchelli (Fig. 18). In contrast, seeds with irregular shape, a strongly wrinkled or crumpled surface coupled with the presence of two tails (one is the funiculus), and/or longitudinal wings characterize four of the five species of sect. Radiata; H. muirii, H. elsiae (Fig. 14), H. radiata (Fig. 1 1), and H. brevifolia (Fig. 12). The latter species appears to be the most specialized in this respect. These feaThis content downloaded from 157.55.39.49 on Tue, 09 Aug 2016 05:26:20 UTC All use subject to http://about.jstor.org/terms 1984] WAGNER & GOLDBLATT-HESPERANTHA 183 TABLE 1. Voucher information for the species of Hesperantha and Geissorhiza studied here. Species of Hesperantha are arranged taxonomically according to the sectional classification proposed by Goldblatt (1982). All collections are from the Cape Province, South Africa. Species Collection Data