Notes on the Systematics of Hesperantha (Iridaceae) in Tropical Africa

Abstract

Three species of Hesperantha are here recognized in tropical Africa: H. petitiana is widespread in highland areas from Ethiopia to eastern Zimbabwe; H. ballMi is a local endemic of the Chimanimani Mountains in eastern Zimbabwe; and H. longicollis occurs in highlands in Malawi and Zimbabwe, extending into eastern Botswana and the Transvaal, South Africa. Hesperantha petitiana is taxonomically complex and includes lowgrowing and small-flowered plants corresponding to Ixia petitiana and L hochstetteriana, and tall and large-flowered forms corresponding to H. volkensii from Mt. Kilimanjaro, and H. alpina from Mt. Cameroun. Hesperantha petitiana is closely allied to the southern African H. baurii complex and is not readily separable from some southern African collections assigned to this alliance. Chromosome numbers are reported for three populations of H. petitiana, all polyploid and either tetraploid or hexaploid, in contrast to all southern African plants so far counted, which are diploids. The genus Hesperantha comprises some 55 species of Iridaceae-Ixioideae, all small perennial corm-bearing geophytes. While it occurs widely in (Fig. 1), species are concentrated in southern Africa. There are some 36 species in the winter rainfall region of the Cape Province (Goldblatt, 1984) and about 20 species in the well-watered areas of coastal and montane eastern southern (Goldblatt, 1982; Hilliard & Burtt, 1979, 1982). Six species of Hesperantha have been recorded in tropical Africa, from Zimbabwe in the south to Ethiopia in the north, but only three are recognized here. These are the closely allied H. ballii Wild and H. /ongicollis Baker (section Radiata), and the unrelated H. petitiana (A. Richard) Baker (section Concentrica), which is variable and often treated as comprising two or more species or varieties. Hesperantha ball/i and H. petitiana are found only in tropical Africa, H. ball/i being a local endemic of the Chimanimani Mountains of eastern Zimbabwe, while H. petitiana occurs in highland areas above 8,000 ft., almost throughout eastern tropical as well as in Cameroun. Hesperantha longicollis is centered in the highveld of the Transvaal and Zimbabwe (Obermeyer, 1980) but it extends into Botswana to the west and Malawi to the north. Both Hesperantha longicollis and H. ball/i are well-defined species, but H. petitiana is variable over its wide range and appears taxonomically complex. Moreover, it does not seem particularly distinctive and is difficult to separate from a complex of southern African species centered around H. baurii Baker. The variation pattern and taxonomy of H. petitiana is dealt with in detail in this paper, while H. ball/i and H. iongicollis are discussed only briefly. Chromosome number has been determined here for three populations of Hesperantha petitiana, the species previously unknown cytologically. A collection from Mt. Kilimanjaro (Puff s.n.) is tetraploid, 2n (4x) = ca. 50. Two more populations are hexaploid, one from Ethiopia (Puff et al. 820911-1/1) 2n (6x) = ca. 72 and the other from Mt. Cameroun (Thomas sub Goldblatt 7272), 2n (6x) = ca. 76. Basic chromosome number in Hesperantha is x = 13 (Goldblatt, 1984) and all of the many other species so far counted, all from southern Africa, have numbers at the diploid level. The counts for H. petitiana are interesting because they are the first reports of polyploidy in the genus and also because the numbers recorded suggest a possible x = 12 rather than 13 as basic for this species. Unfortunately, the high numbers and small chromoI Supported by grant DEB 79-10655 from the United States National Science Foundation. I thank B. L. Burtt for his constructive criticism of the manuscript and Chris Puff and Duncan Thomas for seed of the plants studied cytologically. 2 B. A. Krukoff Curator of African Botany, Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166. ANN. MISSOURI BOT. GARD. 73: 134-139. 1986. This content downloaded from 157.55.39.127 on Sun, 26 Jun 2016 05:32:54 UTC All use subject to http://about.jstor.org/terms 1986] GOLDBLATT-HESPERANTHA 135 somes make it difficult to establish an exact count and so a second base number in Hesperantha remains uncertain. HESPERANTHA LONGICOLLIS Hesperantha longicollis, typical in Hesperantha in being evening blooming, is closely related to the widespread southern African H. radiata, which extends from Namaqualand on the west coast, through the southern and eastern Cape to Swaziland in the eastern escarpment. The two have, in common, a curved perianth tube and unusual floral bracts, the outer of which have margins partly united around the axis. The two species can readily be identified by a series of distinguishing features. In H. longicollis the flower has a longer perianth tube, 18-25(-30) mm long, well exserted from the bracts; the outer bract is united around the axis only near the base; the leaves are relatively long, usually about half as long as the stem or longer, and plane; and the corm tunics are typically spiny below. In H. radiata, the perianth tube is 10-18 mm long, and only slightly exceeds the bracts; the outer bract has margins united around the axis for half to two-thirds its length; the leaves are typically short, about one-third to half as long as the stem, and tend-to be thicker in the midrib area; and, at least in populations from eastern southern Africa, the corm tunics are not spiny below (Goldblatt, 1984) although some southwestern Cape forms do have a corm with a spiny base. Hesperantha longicollis grows in moist habitats, either in vleis, along streams or in seeps, and it blooms at the end of the dry season, typically in August or September. It is most common in the southern African highveld (Fig. 1) and has been recorded from the Transvaal, Northern Cape, extreme eastern Botswana, central and western Zimbabwe, and recently from Malawi, where it was collected by R. K. Brummitt on the Nyika Plateau (Brummitt 10829) flowering in May. This represents a significant range extension into tropical of what has been regarded as essentially a southern species. The species was recently reviewed for Flowering Plants of Africa (Obermeyer, 1980), in which a full description and synonymy were provided. This need not be repeated here; however, it should be noted that of the three synonyms cited, Hesperantha matopensis, H. widmeri, and H. sabiensis, the last does not apply to H. iongicollis. It is a later synonym of H. bulbifera (Goldblatt, 1984), a rare species, unrelated to H. longicollis, found on damp cliffs and waterfalls in the Eastern Cape and Transvaal (Goldblatt,