Pseudohydrosme buettneri Engl., 1892

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 : 9-12

1. Pseudohydrosme buettneri Engl. ( Engler, 1892:456; Engler & Prantl, 1897: 59; Brown in Thistleton-Dyer, 1901: 160; Engler, 1911: 49).— Figs. 3 and 4 .

Holotype: Gabon, Estuaire Province, Libreville “Gabun, Mundagebiet ; Sibange-Farm ” fl. Sept. 1884, Buettner 519 (Holotype B destroyed or mislaid).

Terrestrial herb, rhizome vertical, subglobose, 2.5 cm long, 2.5 cm wide, surface tuberculate, roots fleshy, from along the length of the rhizome. Leaf unknown.

Inflorescence: Cataphylls three or more, 2–13 × 0.9–1 cm; peduncle 3 cm long, colour and indumentum unknown. Spathe 80 cm long, within pale except for the median longitudinal part which is dark purple. Spadix subcylindrical, 7–8 cm long, c. 1.3 cm diam. Female zone obconic 1.3 cm long. Fertile male zone 2 cm long. Appendix (of sterile male flowers) 5 cm long, c. 1.5 cm diam.

Female flowers 4 mm long, ovary globose-ovoid, 3 mm diam., style 1 mm long, slender; stigma bilobed, 1 mm diam., thick; ovules shortly ovoid, solitary in each locule and attached near the base of the septum. Male flowers with stamens 2 mm long, 2 mm wide, usually the two stamens of a flower, appressed to one another, with bilocular subextrorse thecae nearly reaching the stamen apex. Staminodes subprismatic, 4–6-sided, lacking anther thecae and much smaller in diameter than the stamens (description taken from Engler, 1892: 456 and tab. XV).

Phenology: flowering in September.

Local name and uses: none are known.

Etymology: named for the collector of the only known, and type specimen, Oscar Alexander Richard Buettner (Büttner)—(1858–1927), traveller and collector.

Distribution and ecology: known only from Sibang in Libreville, coastal lowland evergreen forest dominated by Aucoumea klaineana Pierre.

Additional specimens: none are known.

Notes: Pseudohydrosme buettneri has the largest inflorescence by far of all known species of the genus, with an 80 cm long spathe (however, 70 cm has been reached for P. gabunensis in cultivation in Australia according to Hay in litt.). The type specimen had lost the top part of the spathe, but dimensions were given by the collector ( Engler, 1892).

The type, and only known specimen was at B, but is reported to be no longer there ( Bogner, 1981). It may have been lost in the allied bombing of Berlin in March 1943, when most of the specimens at B were destroyed. However, the type specimen of P. gabunensis (see below) dating from about the same time, and also housed at B, has survived.

No additional specimens of this species have been found in the 136 years ensuing from collection of the type specimen. Hetterscheid & Bogner (2013) have questioned whether this species is not just a variant of P. gabunensis . However, this seems highly unlikely, because the specimen differs in three independent characters from P. gabunensis (and P. ebo ):

1. the ratio between the female zone and the male zone (of fertile and sterile flowers) differs greatly between the two. In P. buettneri the ratio is 1:4+, while in the other two species it is less than 1:2.

2. in P. buettneri most of the spadix consists of an appendix of sterile male flowers. No such sterile appendix occurs in the other two species. In fact this character is otherwise unknown in the entire Aglaonemateae / Nephthytideae clade

3. in P. buettneri the stamens are paired ( Engler, 1892), while in the other two species the stamens are not reliably paired, but also present in an indistinct ring of three to five.

Additional differences between the species can be seen in the pistil and spadix. The style in P. buettneri is <1/4 the width of the ovary. In P. gabunensis it is 1/2. The spadix of P. buettneri is cylindrical, and unconstricted, while that of P. gabunensis shows a pronounced constriction at the junction of female and male zones, and the male zone attains a greater width than the female zone.

Conservation. Pseudohydrosme buettneri is here assessed as Critically Endangered (Possibly Extinct). This is because it has only been found once, at a single site, in the “Munda region” at Sibang Farm or Plantation, in 1884. At that time Sibang was far outside Libreville and consisted largely of forest, some of which was exploited to produce forest products such as timber and rubber, and cleared to produce agricultural products by Europeans for international commerce for example by the Woermann company ( Cheek, Harvey & Onana, 2011: 45). The Munda is the estuary that forms the eastern edge of the peninsula on which Libreville sits. Tributaries of the Munda drain the Sibang area. Beginning in 1960, the population of Libreville expanded 20-fold, and its footprint expanded. Only a small part of the original forest formerly known as Sibang survived. This part measures about 400 m × 400 m as measured on Google Earth (see further details under P. gabunensis , below) and is now entitled the ‘Sibang Arboretum’. This minute remnant of forest is probably the most visited by botanists in the whole of Gabon because it is immediately adjacent to the site of the current National Herbarium, LBV (M. Cheek, 2002, personal observation). In the unlikely although hoped-for rediscovery of Pseudohydrosme buettneri , the area of occupancy would be expected to be calculated as 4 km 2 using the IUCN preferred gridcells of this size, and the extent of occurrence of the same size. If it should be found anywhere in the vicinity of Libreville it is likely to be threatened by human pressures since most of the population of Gabon is concentrated here.

The Libreville region has the highest botanical specimen collection density in Gabon, with 5359 specimens recorded in digital format. It also has the highest level of diversity of both plant species overall and of endemics ( Sosef et al., 2005). The coastal forests of the Libreville area are known to be especially rich in globally restricted species ( Lachenaud et al., 2013). These authors detail 19 species globally restricted to the Libreville area, of which eight have not been seen recently and which are possibly extinct. Among these is Octoknema klaineana Pierre, a rainforest tree “only collected in the immediate area of Libreville at the beginning of the 20th century, and only once since” ( Gosline & Malecot, 2011). Most of the collections of this possibly extinct species of Octoknema were also, as with Pseudohydrosme buettneri , from Libreville-Sibang, and were mainly made in the period 1896–1912, during the colonial period, before the city expanded to its current extent. The other seven species recorded as globally restricted to the Libreville area and as possibly extinct by Lachenaud et al. (2013): are Ardisia pierreana Taton ( Taton, 1979), Dinklageella villiersii Szlach. and Olszewski ( Szlachetko & Olszewski, 2001), Eugenia librevillensis Amshoff ( Amshoff, 1958), Hunteria hexaloba (Pichon) Omino ( Omino, 1996), Pandanus parvicentralis Huynh ( Huynh, 1986), Psychotria gaboonensis Ruhsam ( Ruhsam, Govaerts & Davis, 2008) and Tristemma vestitum Jacq.-Fél. ( Jacques-Félix, 1986). These species have also not been seen in several decades, or more, in the case of the penultimate species, since 1861.

The explanation for this hotspot of unique species, fast disappearing if not already extinct, at Libreville may be that it has the highest rainfall in Gabon ( Gosline & Malecot, 2011), with c.2.9 m p.a.

It seems likely that Pseudohydrosme buettneri is an additional lost endemic species to the Libreville area, likely rendered extinct by the expansion of the city. Let us hope it is rediscovered in a fragment of forest in the greater Libreville area, although this seems extremely unlikely given that it was the most spectacular species of the genus with by the largest spathe (80 cm long) known in the genus, and that as stated above, the Libreville area is the most intensively botanically surveyed part of Gabon ( Sosef et al., 2005).

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

Figure 4 Pseudohydrosme buettneri (Buettner 519, B). Drawing of flowering plant. (A) Habit, rhizome and inflorescences; (B) spadix after removal of spathe showing from base to apex, female, male and staminodal flowers; (C and D) paired stamens, side view; (E) paired stamens flowers, transverse section; (F and G) staminodes, sterile male flower; (H) female flower, entire, side view; (J) female flower, longitudinal section. Reproduced from protologue, (Engler, 1892: taf. XVII). Drawn by Josef Pohl. Full-size DOI: 10.7717/peerj.10689/fig-4