Inversodicraea tenax (C.H.Wright) Engl. ex R.E.Fr.

Tchouto, P. & Burgt, X. van der, 2017, A synoptic revision of Inversodicraea (Podostemaceae), Blumea 62 (2), pp. 125-156 : 139-140

14. Inversodicraea tenax (C.H.Wright) Engl. ex R.E.Fr.

Inversodicraea tenax (C.H.Wright) Engl. ex R.E.Fr. (1914) 56, t. 11; Engl. (1915) 274; (1926) 462; (1930) 58. — Dicraea tenax C.H. Wright (1909) 125. — Ledermanniella tenax (C.H.Wright) C. Cusset (1974) 275; (1983) 384; (1997) 4, t. 1B; Beentje (2005) 4. — Type: Kolbe 3141 (holotype K; isotype BOL n.v), Victoria Falls, Livingstone Isl., Zambezi River, ‘Grows in the very swirl of the rapids. Plant a bronze colour and a horny consistency, affording foothold to crabs’, fl., mid July 1906.

Etymology. From the Greek tenax meaning, tenacious, clinging, since this species clings to rocks in rapids (as in fact do all species of the genus).

Distribution — Angola, Botswana, DRC, Namibia, Tanzania, Zambia, Zimbabwe.

Habitat & Ecology — Forming colonies densely covering rocks in swiftly flowing water at, or just below, water level, flowering above water level ( Cusset 1997); 350–1200 m altitude.

Conservation — Inversodicraea tenax is not listed on www. iucnredlist.org since it is so widespread and frequent, being known from 11 locations (our analysis of specimens cited by Cusset (1983)), it falls short of the threshold of 10 locations required to trigger threatened status for non-fragmented species under Criterion B, which is most frequently used for assessing plant species. Therefore we here assess it as Near Threatened (NT). Area of occupancy is estimated as 11 km 2 using IUCNpreferred 1 km 2 cells. Hydroelectric projects pose threats to this species: 254 such projects occur in Zimbabwe alone.

Additional specimens. ANGOLA, Moxico, waterfalls near Luso [Luena], Thompson 50 (J n.v.) . – BOTSWANA, Chobe River, 2 miles N of Kazane , 13 July 1947, Pole­Evans 4214 (K, PRE n.v) . – DRC, Dilolo, River Luan , rocks in fast water, 20 July 1932, R.G.N. Young 184 ( BM) ; ibid., 185 ( BM) ; Vallée de la Eofai, au Nord de Lukafu , sur roches ... a sec ou peu immergeuse pendant les derniers mois de le saison seche, 17 July 1948, A. Schmitz 1833 ( BM) . – NAMIBIA, Caprivi Strip, Zambezi River, Mambova rapids, West 3702 ( SRGH n.v.) . – TANZANIA, Stromgebeit des Oberen Ruhudje. Landschaft Lu- pembe,nordlich des Flusses , Aug.1931, Schlieben 1131A ( BM,K) . – ZAMBIA, Mwinilunga, Kabompo River where it crosses Mwinilunga-Solwezi Rd , mile 69, 17 Sept. 1952, F. White 3282 ( BM) ; ibid., 23 Oct. 1969, Drummond & G. Williamson 9262 (P00179370 image) . – ZAMBIA / ZIMBABWE BORDER, Victoria Falls, Zambezi River, wet rocks in running water, Worsdell sin, July 1909 ( BM) ; ibid., Cataract Island , July–Aug. 1919, Burtt­Davy 17827 ( BM) ; ibid., July 1926, E.L.Stephens s.n. ( BM) ; ibid., June 1937, Obermeijer 36515 ( BM) ; ibid., 1 Aug. 1941, Greenway 6255 ( BM, K) ; ibid., just above falls on basalt rocks,abundant,flowering just above the water, 17 Aug. 1947, Brenan 7729 ( BM, K) ; ibid., Livingstone Island , 21 Nov. 1949, without moss-like growth, Wild 3129 ( BM) ; ibid., 3000', Eastern Cataract , 6 July 1953, Schelpe 3928 ( BM) .

Notes — With I. ledermannii the most widespread and frequent Inversodicraea species.

It occurs in every country south of DRC and north of S. Africa except Malawi and Mozambique. The only other species of the genus recognised in South-Central Africa is I. torrei ( Mozambique, a single location). The projections from the dorsal leaflet surfaces of I. tenax distinguish it from most other species and otherwise only occur in I. xanderi , I. harrisii , and I. congolana . The type species of the genus, first collected on the Zambesi River at the Victoria Falls on the border of Zambia and Zimbabwe (Kolbe 3149, the type location is mistakenly given as ‘Tanzanie’ by Cusset 1983: 384). We have counted 27 collections made at the type locality from those cited for the species by Cusset (1983). This exceeds all other species of the genus for number of records at one site. By error, these same 27 specimens are all attributed to Tanzania by Cusset (1983).

Curiously, the species had not previously to this paper been reported from DRC, neither by Cusset (1983, 1997) nor by Hauman (1948).

Inversodicraea tenax as currently defined is morphologically variable with discordant elements, effectively comprising a species complex that would reward further analysis.