Stenostephanus Nees

Wood, J.R.I. Stenostephanus (Acanthaceae) in Peru. Kew Bulletin 74: 64 doi:10.1007/s12225-019-9843-0

Distribution

Stenostephanus Nees is an entirely neotropical genus of around 80 species. It is distributed along mountains from south central Mexico through Central America and then along the Andes from Venezuela south to the Santa Cruz region in central Bolivia. A single species, S. lobeliiformis Nees occurs in the Atlantic Forest of southeastern Brazil. The greatest diversity is found in Colombia from where 29 species are known (Wood 2009, 2016). There are accounts of the genus in Bolivia (Wasshausen 1999a; Wasshausen & Wood 2004), in Ecuador (Wasshausen 2013) and in Colombia (Wood 1988, 2009) but no account of the genus has ever been published for Peru.

Morphology General Habit

Stenostephanus species are herbs or weak subshrubs with decumbent, ascending or erect stems up to about 3 m in height

Morphology General Cystoliths

Cystoliths are present, particularly on the leaves but sometimes also on other vegetative parts

Morphology Stem

Stems are often swollen at the nodes

Morphology Leaves

Leaves are entire to undulate, usually equal to slightly unequal in each pair but strongly anisophyllous plants are apparently unknown

Morphology Reproductive morphology Inflorescences

The inflorescence is commonly a terminal thyrse but small cymes or thyrses may arise in the upper leaf axils

Morphology Reproductive morphology Inflorescences Bracts

Prominent bracts may be present at the base of the thyrse but diminish in size upwards at each branching point

Morphology General Indumentum

The whole inflorescence may be glabrous or glandular-hairy, the glandular hairs often developing as the inflorescence matures

Morphology Reproductive morphology Flowers Calyx

The calyx is divided to near the base into five equal, usually narrowly lanceolate lobes, these somewhat accrescent in fruit

Morphology Reproductive morphology Flowers Corolla

The corolla may be tubular, ± suburceolate with four short lobes, or more or less 2-lipped with a simple upper lip and a 3-lobed, often spreading lower lip In colour, the corolla may be blue, yellow, red, purple, pink or bicoloured

Morphology Reproductive morphology Flowers Androecium Stamens

The androecium consists of two, usually exserted stamens, inserted above the basal part of the corolla tube, each stamen with a monothecous anther

Morphology Reproductive morphology Flowers Pollen

The pollen is of a characteristic shape resembling an aeroplane wheel

Morphology Reproductive morphology Flowers Gynoecium

The gynoecium consists of a narrowly oblong ovary and single style with a very small stigma

Morphology Reproductive morphology Fruits

The fruit is an oblong 4-seeded, stipitate capsule, the seeds discoid or lenticular, often tuberculate.

Ecology

Stenostephanus is a characteristic cloud forest species with a few species occurring in lower altitude rain forest but, apart from S. longistamineus, always on or near mountainous regions. Most species are very local endemics, often known from a single locality or with a very restricted range. Many appear to flower irregularly, perhaps in response to particular climatic conditions. One Mexican species is confirmed to be plietesial (Daniel 2006) but it is quite probable that other species are also plietesial. Species with a tubular red flower (the majority of species in Peru) are presumably hummingbird-pollinated but very little reliable information is known about the pollination of these plants. Species with strongly 2-lipped corollas tend to occur at higher altitudes (1500 – 2500 m) than those with a subcylindical corolla.

Note

In morphology, pollen and ecology, Stenostephanus is very similar to the African genus, Brachystephanus Nees and it is difficult to see how the two genera can be consistently distinguished. Champluvier & Darbyshire (2009) compared the two genera in some detail but decided that no attempt to unite them should be made before further molecular studies were made. Stenostephanus is the older name so there is no threat to the nomenclatural stability of the neotropical species.