Isoglossa nervosa C.B.Clarke

Darbyshire, I., Pearce, L. & Banks, H. Kew Bull (2011) 66: 425. https://doi.org/10.1007/s12225-011-9292-x

Morphology General Habit

Perennial herb, erect or ascending to weakly so, 30 – 150 cm tall, lowermost stems often trailing and rooting at the nodes

Morphology Stem

Stems quadrangular when young, sulcate between the angles, with two opposite lines of pilose or antrorse-pubescent hairs, sometimes restricted to immediately below the nodes, later (rarely early) glabrescent

Morphology Leaves

Leaves (excluding uppermost pairs) ovate or elliptic, 4.5 – 12 × 2.3 – 6.5 cm, base attenuate or with a narrowly cuneate wing-like extension, apex acuminate or attenuate, midrib above, margin and veins beneath usually pubescent, often with scattered short hairs between the veins above, these appearing deflated in dried material, sometimes with few short glandular hairs beneath, rarely largely glabrous; lateral veins (5 –) 6 – 9 pairs; petiole 1 – 6 cm long, usually conspicuously pilose above, rarely glabrous; uppermost pairs of leaves broadly ovate with a cordate base, often sessile and ± reduced

Morphology Reproductive morphology Inflorescences

Inflorescence terminal (often also with reduced lateral inflorescences), a lax panicle, pyramidal to narrowly so in outline, 9 – 40 × 2.5 – 20 cm, many-branched at maturity, branching along main axis usually opposite, cymules sometimes partially secund; axes puberulous and glandular, at first the glands sparse and short-stalked, becoming ± densely glandular-pilose or -pubescent at maturity, hairs then 0.5 – 1 mm long, gland tips capitate, axes often also with scattered or numerous long wispy eglandular hairs to 2.5 mm long; main axis bracts foliaceous in the lower portion or at the lowermost node only, ovate to lanceolate, 7 – 32 × 3 – 16 mm, then  ±  gradually (rarely rapidly) reducing in size upwards, those at the midpoint of the axis lanceolate, subulate or narrowly elliptic, (1.5 –) 2.5 – 20 × 0.5 – 4.5 (– 8) mm, apex acute; bracteoles linear-lanceolate, 1 – 3 mm long; pedicels 0.5 – 1.5 mm long

Morphology Reproductive morphology Flowers Calyx

Calyx lobes linear-lanceolate, 2 – 6 mm long in flower, extending to 3.5 – 6.5 mm in fruit, indumentum as axes

Morphology Reproductive morphology Flowers Corolla

Corolla 11.5 – 19 mm long, white or pink, with red or pink markings on palate of lower lip; shortly pubescent externally at least on the dorsal side, glabrous internally; tube campanulate or narrowly so, 5 – 8.5 mm long, 1 – 2 mm wide at base, ventral side declinate, sometimes slightly ventricose beyond attachment point of stamens, (2.3 –) 3 – 5 mm wide at mouth; upper lip triangular, 4 – 6.5 mm long, lobes reflexed, 1 – 2 mm long; lower lip somewhat protruding, (5 –) 6.5 – 11 mm long, lobes 2 – 5 mm long, palate raised with a central groove, glabrous

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens attached in distal half of or midway along the corolla tube; filaments free for 3 – 4.5 mm, glabrous; anther thecae superposed, slightly overlapping or separated by up to 0.6 mm, subparallel, each 0.9 – 1.4 mm long

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary glabrous or with few minute hairs; style (4.5 –) 5.5 – 7.5 mm long, glabrous or pubescent in lower half; stigma ellipsoid, 0.15 – 0.25 mm wide

Morphology Reproductive morphology Fruits

Capsule 9 – 11 mm long, puberulous and often with scattered short-stalked glands and/or long eglandular hairs

Morphology Reproductive morphology Seeds

Seeds 1.7 – 2.4 × 1.2 – 1.6 mm, with elongate glochidiate tubercles most dense towards the rim.

Distribution

Endemic to the Cameroon Highlands, recorded in SE Nigeria, Bioko and W Cameroon.

Ecology

Recorded from montane forest of Podocarpus, Prunus, Myrsine, Nuxia and Syzygium, bamboo thicket, upper forest-grassland transition thickets and in adjacent montane grassland; alt 2000 – 3000 m. It almost certainly has a plietesial ecology (see Introduction).

Conservation

This species is listed by IUCN (2009) as Vulnerable (VU D2) based upon the assessment in Cable & Cheek (1998). At that time, it was considered to be endemic to Mt Cameroon. It is now known to be more widespread in the Cameroon Highlands, being present on most of the high peaks, with eight localities known and an AOO estimated at 32 km2. However, whilst the Mt Cameroon and Mt Oku sites appear to be secure from disturbance, those on the BamboutosMts and the Mambilla Plateau are under great threat of clearance from agriculture, if not already lost. A further potential threat to whole populations is through stochastic events during the long maturity cycle of this plietesial taxon, for example fire-damage at the seedling stage. It is therefore reassessed here as Vulnerable (VU B2ab(iii, iv) + c(iv)).

Note

This species is closely allied to Isoglossamembranacea C. B. Clarke, described from Malawi. Clarke (1900) in fact described both species in the same publication but had seen only flowering material of each and he did not remark upon their similarity. In fact, he rather unhelpfully keyed out the west African species purely on geography with no supporting characters. With fruiting material now available, the affinity is clear. They share hairy capsules, seeds with elongate-glochidiate tubercles (Fig. 2C & D), similar inflorescence indumentum and corolla shape, and leaves with a cuneate-attenuate base which can form a wing along the upper petiole (although often less pronounced in I. nervosa).

Two subspecies are currently recognised in I. membranacea: subsp. membranacea from the Misuku Hills of N Malawi and subsp. septentrionalis I. Darbysh. from S Sudan and NE Congo-Kinshasa through Uganda to W Kenya (Darbyshire2009). Rather surprisingly, I. nervosa appears closest to the former. Well-developed specimens of I. nervosa are readily separable from those of I. membranacea by having one or more of the following characters: larger corollas, larger and leafy main axis bracts in the lower portion of the inflorescence and/or the stems being hairy when young (see key below). In addition, the leaf veins are typically more prominent beneath in I. nervosa. However, all these characters are rather variable and no single one of them is wholly diagnostic in isolation. Small (perhaps immature?) plants of I. nervosa can appear very close to I. membranaceasensustricto and Jacques-Felix 9092 from Adamoua, Cameroon is largely inseparable. Isoglossa nervosa could therefore be considered a further subspecies of I. membranacea but we keep them separate here based on the notable differences in the majority of the material. Whilst the disjunction is great, I. nervosa occupies largely the same habitat as I. membranacea, that of very wet Afromontane forest.

IUCN Red List of Threatened Species https://www.iucnredlist.org/species/39469/2926941

Conservation

VU - vulnerable

Acanthaceae, H. Heine. Flora of West Tropical Africa 2. 1963

Morphology General Habit

Stems fairly stout, ribbed, rather densely glandular-hairy on the young parts, 5 ft. high

Morphology Leaves

Coarser strongly-nerved leaves

Morphology Reproductive morphology Flowers

White flowers, turning reddish.