Polyarrhena Prostrata

''Polyarrhena'' is a genus of low, branching shrublets that is assigned to the daisy family (Compositae or Asteraceae). Its stems are alternately and densely set with entire or somewhat toothed leaves. Like in almost all Asteraceae, the individual flowers are 5-merous, small and clustered in typical heads, and which are surrounded by an involucre of in this case three whorls of bracts. In ''Polyarrhena'', the centre of the head is taken by yellow disc florets, and is surrounded by one single whorl of white ligulate florets that have a pinkish-purple wash on the underside. These florets sit on a common base (or receptacle) and are not individually subtended by a bract (or palea). The species occur in the Cape Floristic Region. ''Polyarrhena reflexa'' has long been cultivated as an ornamental and is often known under its synonym ''Aster reflexum''.

Taxonomy

In his famous work Species Plantarum, Carl Linnaeus described ''Aster reflexus'', which was the first valid scientific description of a species that is now assigned to the genus ''Polyarrhena''. Carl Peter Thunberg thought this species would be better placed in ''Chrysocoma''. In the ''Dictionnaire des Sciences Naturelles'', volume 56, that was published in 1828, French Asteraceae specialist Henri Cassini erected the genus ''Polyarrhena'', and named the type species ''Polyarrhena reflexa'', that was based on ''Aster reflexus''. The genus name is a contraction of "many", and "male", which Cassini chose because he said it fitted this genus. Christian Friedrich Lessing in 1832 assigned ''P. reflexa'' to his new genus ''Elphegea'', with two other species that have sterile disc florets, ''E. bergeriana'' (now ''Felicia bergeriana'') and ''E. ciliata'' (now ''Zyrphelis taxifolia''), Nees von Esenbeck in the following year retained ''Polyarrhena'', but Augustin Pyramus de Candolle included ''P. reflexa'' in '' Felicia'' in 1835, while Irish botanist William Henry Harvey in 1865 reenlisted the species under ''Aster''. Jürke Grau in his review of the genus extended ''Polyarrhena'' by reassigning ''Felicia imbricata'', and describing two new species, '' Polyarrhena prostrata'' and '' P. stricta'', and two new subspecies, ''P. reflexa'' subsp. ''brachyphylla'' and ''P. prostrata'' subsp. ''dentata''.

Description

The species belonging to the genus ''Polyarrhena'' are shrublets, creeping with up to long branches in ''P. prostratum'', or with ascending to erect stems of between in ''P. imbricata'', in ''P. stricta'', in ''P. reflexa''. Three types of hairs can be found on these plants, stiff bristles with several cells at their base, thin hairs consisting of several rows of cells and short glandular hairs. The branches are mostly densely set with alternate leaves entire or weakly toothed margins, densely hairy or soon becoming hairless. The heads stand individually at the tip of side branches, which sometimes themselves occur in clusters. A leafless inflorescence stem (or peduncle) may be present or absent. The centre of the heads is taken by only male, only bisexual, yellow disc florets or a mixture of the two, which is encircled by one row of female white ligulate florets, that have a pinkish purple wash on the reverse, have a pointy tip without teeth. The style has triangular appendages. The dull brown to yellowish, indehiscent one-seeded fruits (called cypselas) have no short hairs over their surface, have two vascular bundles along their edges, are crowned by one row of bone-colored, soft hairs (or pappus)with short teeth along their length that easily detach. Near the tip of the cypsela is also a hornlike ring of about high.

File:Polyarrhena imbricata Rebelo 2.jpg, ''P. imbricata''
File:Polyarrhena prostrata Rebelo 1.jpg, ''P. prostrata''
File:Polyarrhena reflexa Rebelo 2.jpg, ''P. reflexa''

Differences between the species

''Polyarrhena stricta'' is an ascending or erect low shrublet of about high with the outer whorl of involucral bracts hairless in the lower third or with some stiff hairs, narrowly ovate or lancet-shaped ascending leaves. ''P. reflexa'' subsp. ''reflexa'' is an ascending or erect high shrublet of about high with the outer whorl of involucral bracts hairless in the lower third or with some stiff hairs, broadly ovate reflexed leaves of long. ''P. reflexa'' subsp. ''brachyphylla'' is an ascending or erect high shrublet of about high with the outer whorl of involucral bracts entirely hairless, broadly ovate reflexed leaves of up to long. ''Polyarrhena imbricata'' is an ascending or erect low shrublet of about high with the outer whorl of involucral bracts over the entire surface consistently hairy, the leaves pressed to the stem, overlapping and the heads sit on up to long leafless peduncles. ''P. prostrata'' subsp. ''prostrata'' is a creeping, copiously branching shrublet with branches up to long with the outer whorl of involucral bracts over the entire surface consistently hairy, the densely hairy, ovate leaves with entire margins, at right angles to the branches or even somewhat reflexed. ''P. prostrata'' subsp. ''dentata'' is also a creeping, copiously branching shrublet with branches up to long with the outer whorl of involucral bracts over the entire surface consistently hairy, and also the roughly hairy leaves at right angles to the branches or even somewhat reflexed, but these are narrow ovate to lancet-shaped, tend to lose these on the surface and have a margins with teeth.

Convergent evolution

The species of ''Polyarrhena'' have a strong likeness to those of '' Felicia'' section ''Anhebecarpaea'', in particular ''P. reflexa'' to '' F. echinata'', ''P. imbricata'' to '' F. nordenstamii'' and ''P. stricta'' to '' F. westae''.

Conservation

For two taxa, ''Polyarrhena imbricata'' and ''P. reflexa'' subsp. ''reflexa'' the continued survival is considered to be of least concern. Both subspecies of ''P. prostrata'' are considered rare. ''P. reflexa'' subsp. ''brachyphylla'' is thought to be a vulnerable species. For ''P. stricta'', insufficient data are available to assess its conservation status.

References

{{Taxonbar, from=Q577300

Endemic flora of South Africa
Astereae
Asteraceae genera