PTERIDOPHYTES OF THE OKAVANGO DELTA , BOTSWANA ( SOUTHERN AFRICA )

VEGA HERNANDEZ, E. & R. CAUDALES (2001). Pteridophytes of the Okavango Delta, Botswana (Southern Africa). Anodes lard. Bot. Madrid 58(2): 311-323. The Okavango Delta in Botswana constitutes one of the larger inland deltas in the world. Fifteen species of fern were found in this ecological system integrated into 10 genera and 9 families. Included in this report are descriptions of each family, genus and species. Also given are artificial keys to the genera, as well as keys for genera and species within the family when necessary. A documented distribution and ecological notes for each species also appears.


INTRODUCTION
the SE in the Kunyere and Thamalakane Faults, Botswana.The delta forms a unique Okavango, one of the larger African rivers, environment comprised of 6,000 km 2 of has as its head waters the Cuito and Cubango permanent swamps, 10,000 km 2 of permanent Rivers in N Angola.These rivers flow from islands or peninsulas with dry savanna, and the highland of Planalto Central in N Angola 10,000-12,000 km 2 of seasonal swamps and join in S Angola to form the Okavango (MCCARTHY & al, 1993).River.In Botswana the Okavango River runs The geological composition of the Delta is between two parallel faults in the Panhandle made up of rocks composed of granitoic which control the direction of the river (Ross, gneiss of the Archean Basement Complex. 1992).Close to the end of the faults the The strata are variably metamorphosed, Okavango is divided into three major rivers comprized of quartz schists, quartzites and forming a broad delta.Due to the low slope dolomitic marbles of the late Pre-Cambrian of the Kalahari Desert, the Delta starts S of age (HUTCHINS & al, 1976).the Caprivi strip in Namibia and dies out at The climate of the Okavango Delta is semi-arid with an average rainfall of 500 mm y 1 (SILITSHENA & MCLEOD, 1992).Water level is affected by the rainy season in Botswana between November and March, and by a flooding period from May to July due to the rainy period in central Angola.
VEGETATION OF THE OKAVANGO DELTA The Okavango Delta (SMITH, 1976) comprises several associations in a complex of permanent swamps, open water, seasonal swamps and floodplain.The permanent swamps are always under water and never dry out.The floodplains are areas liable to inundation, which when flooded, remain so only for part of the year.The seasonal swamps are in an intermediate category.
Floodplains contain mostly grasslands covered by Hyparrhenia rufa, Cymbopogum excavatus as well as species of Vossia, Echinocloa, Leersia, Oryza and Paspalum.Lastly, the islands and mainland edges are usually marked by a tree line composed of species of Hyphaene petersiana, Phoenix reclinata, Ficus verriculosa and others.
Several studies of regional floras have been published on the Pteridophytes of Southern Africa (BURROWS, 1990;SCHELPE, 1970;LAUNERT, 1968LAUNERT, ,1975; SCHELPE & ANTHONY, 1986).However, there are few publications on the flora specifically of the Okavango Delta in Botswana (SMITH, 1976), and what has been described is scattered in the scientific literature.There is yet no comprehensive study of the ferns of the Delta.Four new records of ferns for Botswana have been reported recently, including Marsilea minuta L. found in the Okavango Delta (VEGA HERNANDEZ & al, 1997).The present report describes the ferns of the Okavango Delta, including information on their habitat and ecology, as well as map distributions.

METHODS
A series of explorations were carried out in the Okavango Delta, Botswana, with the objective of searching specifically for ferns.Regional herbaria were consulted as well, including the Herbarium of the University of Botswana, the Herbarium of the National Museum of Gaborone, Botswana, the National Botanical Institution Herbarium of Pretoria, South Africa, National Herbarium and the Botanic Garden of Harare, Zimbabwe.Scientific consensus classification of Pteridophyta was taken from HENNIPMAN (1996).The pteridologycal flora of the Okavango Delta is integrated into 7 families, 10 genera and 15 species.A genus with nearly of 250 species, mostly tropical.One species occurs in the Okavango Delta.

V. MARSILEACEAE
Fronds circinate in bud, long-stipitate and 2-4 foliate, segments with flabellate venation.Fertile plants bearing one or more sessile or stalked capsule-like sporocarps at the base of the stipes or along their lower part.Sporocaps variously septate within, the sori solitary within the compartments, each sorus with two kinds of sporangia: megasporangia each containing a single large megaspore with a mucilage coating, and microsporangia containing numerous minute microspores.
A family with three genera and about 56 species (PROCTOR, 1989).One genus in the Okavango Delta.

See characteristics of the family.
A genus with about 50 species of worldwide distribution (PROCTOR, 1989).Four species occur in the Okavango Delta.Launert in Mitt. Bot. St. Samml., Munch. 3: 505 (1960) Rhizome stout, strongly branched.Stipes of the floating form up to 250 mm; leaflets obovate to obdeltate, distal margins entire to slightly irregular, glabrous; stipes of the dry land form up to 200 mm long; le_aflets narrow, minutely pilose to glabrous.Sporocarps growing downward, brown to dull, densely pilose; pedicels free, arising from the axis; superior tooth short, conical, obtuse; inferior tooth absent.

VI. SALVINIACEAE
Small floating plants with dorsoventral structure.Stems horizontal, bearing fronds in whorls of three, two of them green, entire, the third leaf submerged, finely dissected and hairy.Floating fronds with a reticulate venation, the upper surface covered by erect papillae, the under-surface with septate hairs.Plants monoecious, sori in clusters on the submerged frond.
A family with one living genus.

Salvinia S6guier
A widely distributed genus of about ten species (PROCTOR, 1989).Only one species occurs in the Okavango Delta.Rhizome horizontal, bearing fronds in whorls of three, two of them floating, the third one submerged.Floating leaves up to 25 mm, obovate to orbicular, upper surface covered by multicellular hairs, lower surface sparsely covered by hairs; submerged leaf finely dissected into several root-like segments, bearing sporocarps.Sporocarps spherical to ovoid arranged in two rows.
Areas of free waters or forming great mats on still water.This species is one of the most important aquatic weeds in S Africa and its populations in Botswana needs to be controlled.Introduced from tropical America to C and S Africa.Also India, Sri Lanka, Australia and Indonesia.OKAVANGO DELTA: Recorded in Chove River and Kasane.Vouchers: Type, Mitchell 1330 (SRGH, holo!, PRE, iso!), Smith 7i65(GAB,SRGH)(fig.9).

VII. AZOLLACEAE
Small or minute, floating or subaquatic, heterosporous plants.Growth habit dorsoventral, stems branching pinnately or dichotomously, baring stems and roots.Fronds imbricate, alternate in two rows, each frond divided into two lobes, the lobes opposed, unequally developed, the upper ones covering the stem, with a cavity filled with mucilagenous material containing filaments of symbiotic cyanobacteria, the lower ones submerged, without cavities.Sori born in pairs on submerged frond lobes, each pair consisting of two microsporocarps, or two megasporocarps.
A family with a single genus.

Azolla Lam.
A widespread genus of seven species (LUMPKIN & PLUCKNETT, 1982).A single species occurs in the Okavango Delta.

Free
waters of pools, back-waters of rivers and permanent swamps, in full sunlight or slight shade.Tropical Africa, from Gambia and Senegal to Kenya and Tanzania, and from Chad and Sudan to South Africa (Transvaal) and Botswana.Also in Madagascar.OKA-VANGO DELTA: Recorded in the Kwando River (Caprivi Strip) in Namibia, in the Botswana border, and in several permanent swamps of the Okavango Delta.Vouchers: Edwards 4317 (PRE), Gibbs Russell 2386 & 2817 (PRE), Smith 209,2678 (PRE, SRGH) (fig.9).
Sori located near margin or in reflexed flaps .. 2 articulated.Sporangia marginal, borne along 2. Sori on the under side of sharply reflexed ^ ^^ yeins of on ^ underside of me marginal lobes, venation dimidiate or flabellate " ,• .«• L , i _ 4< Adiantum reflexed mar g in of me lobes --Sori near margin, venation free or anatomosing A cosmopolitan genus of over 200 species, 3 specially in South America.One species 3. Ultimate fertile segment with a distinct entire to occw& in ^ okavango Delta.
R. Br., Prodr.Fl.Nov.Holl.:163(1810)ZambeziValley of Zimbabwe and Mozambique, Malawi, Zambia.Widespread in tropical Africa, S Asia and Australia.OKAVAN-GO DELTA: Recorded in the Kwando River (Kwando Hunters).Voucher: Smith 2217 (PRE)(fig.2).Areas of free water, floating in quiet waters or rooted in mud.South Africa (Natal), Swaziland, Botswana, Mozambique, Zimbabwe, Angola, Zambia and tropical Africa, including W Senegal and N Sudan.Also Stipes of the floating form up to 35 mm long; leaflets variable, distal margin sinuate; stipes of the dry land form up to 60 mm long, leaflets small, margin entire, shallowly irregular.Sporocarps usually single, variable in shape, dark black, usually shiny, with the epidermis peeling off with age; pedicels free, short, 1. Sporocarpus dark black, outer layer peeling off with age 1. M. nubica -Sporocarps brown to dull 2 2. Pedicels branching repeatedly producing tight clusters of sporocarps with both teeth prominent 2. M. niinuta Seasonally flooded ponds, pans and woodland savannas, usually on sandy soils.Namibia, Botswana and Zimbabwe.OKAVANGO DELTA: Recorded near the Boteti River.