Pseudohydrosme Engl., 1911

Cheek, Martin, Tchiengué, Barthélemy & van der Burgt, Xander, 2021, Taxonomic revision of the threatened African genus Pseudohydrosme Engl. (Araceae), with P. ebo, a new, critically endangered species from Ebo, Cameroon, PeerJ 9, pp. 1-32 : 6-9

Pseudohydrosme Engl. ( Engler, 1892: 455; Brown in Thistleton-Dyer, 1901: 160; Engler, 1911: 47; Mayo, Bogner & Boyce, 1997: 221–222)

Type species: Pseudohydrosme gabunensis Engl. (Lectotypified by N. E. Brown in Thistleton-Dyer, 1901: 160).

Zyganthera N. E. Br. (Brown in Thistleton-Dyer, 1901:160). Heterotypic synonym.

Type and only species: Pseudohydrosme buettneri Engl. ( Engler, 1892) .

Large, seasonally dormant, monoecious herbs. Rhizome shallowly subterranean, the growing point at ground level, subglobose or cylindrical with annular leaf scars, and erect to horizontal or obliquely inclined, growing continuously and not renewed with each growing period. Roots fleshy, produced along length of rhizome, sometimes reproductive (the distal ends rising to the surface and producing new plants). Leaf solitary, large, emerging from several cataphylls; petiole cylindrical, erect, long, with minute and sparse prickles, sheath very short and inconspicuous. Blade transitioning from simple, sagittate and entire in seedlings, leaves of older plants developing slits and divisions (see above), in mature plants leaves dracontoid: trisect, primary divisions pinnatisect, distal lobes mostly truncate and bifid, sessile and decurrent, proximal lobes acuminate; primary lateral veins of ultimate lobes pinnate, often forming a regular submarginal collective vein ( P. ebo ) or an irregular collective vein, or veins running into margin (often in P. gabunensis ), higher order venation reticulate.

Inflorescence solitary, appearing separately from the leaf. Cataphylls paperymembranous, (3–)4–6, proximal ± triangular, small, distal oblong-elliptic, exceeding spathe tube, pink-brown or red-brown, sometimes ( P. gabunensis ) spotted white. Peduncle concealed by cataphylls at anthesis, terete, very short <1/10th the length of the spathe, with minute, sparse, prickles. Spathe large, fornicate, unconstricted/very broad, with flaring auriculate margins; tube convolute, fleshy, obconic, with a few sparse prickles on the outer surface proximally. Spadix short, about 1/10–1/4 length of spathe, sessile, female zone subcylindric ( P. ebo ) obconic ( P. buettneri ) or gently obconic or conic ( P. gabunensis ), male zone cylindric, obtuse or rounded, subequal to ± twice (±four times in P. buettneri ) as long as female, completely covered in flowers and fertile to apex ( P. gabunensis ) or with a distal appendix twice as long as the fertile portion and covered in sterile male flowers ( P. buettneri ) or flowers only laxly covering the spadix axis in the female zone and with the distal part of the axis of the female zone completely naked in places ( P. ebo ).

Flowers unisexual, perigone absent. Male flower 2–5-androus, stamens free, subprismatic, compressed, anthers sessile, connective thick, broad, overtopping thecae, thecae oblong, long, lateral, dehiscing by apical pores. Pollen extruded in strands, inaperturate, ellipsoid-oblong, very large (mean 106 micrometres diam.) exine psilate to slightly scabrous. Sterile male flowers ( P. buettneri ) composed of subprismatic, free staminodes. Female flower ovary globose to broadly ellipsoid, usually prismatic, 2–3- locular, ovules 1 per locule, anatropous, funicle short, placentation axile, at base of septum, stylar region attenuate to cylindric, narrower than ovary, stigma thick, shallowly 2–3- lobed or subdiscoid, concave centrally, wet when receptive.

Infructescence with slightly accrescent peduncle. Berry at first white, ripening dark purple, fleshy, wrinkled when mature, oblong-ellipsoid, laterally compressed to slightly bilobed, stipitate, large; stigma and style persistent (known only in P. gabunensis ). Seeds subglobose to broadly ovoid, one side convex, the other slightly flattened, testa thin, whitish, smooth, papery, transparent; embryo large, outer surface green, inner white, endosperm absent, raphe distinct, hilum and micropyle purple, plumule with leaf primordia. Three species.

This description is based on that of Mayo, Bogner & Boyce (1997), with the addition of descriptions of the fruit, seed and roots of P. gabunensis , mainly from ( Hetterscheid & Bogner, 2013), with novel features of the nervation, spathe and spadix from P. ebo (below in this article)

Phenology: flowering September and October (or March in cultivation in Europe); in leaf Dec.-April.

Distribution and habitat: Cameroon, Gabon and Congo(Brazzaville), lowland evergreen forest on coastal sediments (Gabon) or inland foothills on basement complex rocks (Cameroon) ( Fig. 3 ).

Etymology: meaning “false Hydrosme”, Hydrosme Schott is a synonym of Amorphophallus .

Local name and uses: none are documented.

Conservation: all species are highly infrequent and globally threatened according to IUCN (2012) criteria (see species accounts below), and P. buettneri is possibly extinct (not seen for over 100 years, the majority of its former habitat destroyed).

Pollination in the wild has not been investigated in detail in Pseudohydrosme , but is almost certainly by insects as is usual in Araceae . Two different species of flies, and two of beetles were reported to visit P. gabunensis (see below). In cultivation the stigmas are reported to be wet and receptive for only two days, and the scent reported to be faint, of lettuce (Lactuca) in the same species (see below also). Following successful fertilisation, seed development is reported to take up to 10 months in P. gabunensis (see below). Seed dispersal is probably by either ground-dwelling mammals or birds consuming the thinly fleshy purple berries.

DNA analysis was performed by Cabrera et al. (2008) for Pseudohydrosme gabunensis , using five regions of coding (rbcL, matK) and noncoding plastid DNA (partial trnK intron, trnL intron, trnL–trnF spacer). These sequences were subsequently used by Cusimano et al. (2011) and Nauheimer, Metzler & Renner (2012). The voucher is Wieringa 3308 (WAG), identified by Hetterschied, GenBank codes are AM905760 , AM920582 , AM932319 + AM933315 .

Cultivation of one species, Pseudohydrosme gabunensis is unknown in Africa yet widespread but infrequent in the tropical glass-house collections of several large extra-African botanical gardens, mainly in Europe, Australia and N. America (see under that species).

Chromosome numbers are reported of one species, Pseudohydrosme gabunensis , as 2n = ca. 40 ( Mayo, Bogner & Boyce, 1997; Bogner & Petersen, 2007).

Germination in Pseudohydrosme gabunensis is cryptocotylar and takes 3 weeks to 10 months. The large seed embryo remains buried, producing a single hastate seedling leaf ( Hetterscheid & Bogner, 2013). The seedling type is C2 in the classification of Tillich (2014).

Medicinal uses, and chemistry is unreported in Pseudohydrosme . However, the much more frequent sister genus Anchomanes , is harvested as a traditional medicine for example in Cameroon, and contains bioactive compounds ( Cheek, 1992)

Identification key to the sections and species of Pseudohydrosme

1. Spadix with distal half covered in sterile male flowers. Sect. Zyganthera .......................................................... 1. P. buettneri

1. Spadix lacking sterile flowers, distal part with fertile male flowers only.................................................. Sect. Pseudohydrosm e …2

2. Male and female zones of spadix contiguous; entire spadix covered in flowers densely arranged in both male and female zones; spathe blade inner surface yellow, greenish yellow or white with abrupt transition to a central dark red area; stigmas 2(–3)-lobed. Gabon (probably Congo-Brazzaville)........... 2. P. gabunensis

2. Male and female areas of spadix incompletely contiguous; female flowers laxly arranged with axis of female zone partly naked especially distally; spathe blade inner surface light reddish brown or pink, with wide green veins, very gradually becoming darker towards the centre; stigmas 3(–2)-lobed. Cameroon............................... 3. P. ebo

Figures

Figure 3 Global distribution of the species of the African genus Pseudohydrosme. Red dot = P. buettneri; black dots= P.gabunensis (location not available for Congo specimen); blue dot = P. ebo. Full-size DOI: 10.7717/peerj.10689/fig-3