Neotropical Apocynaceae (Asclepiadoideae)

David Goyder

Royal Botanic Gardens, Kew, U.K. 

Description

Large shrubs (Calotropis, introduced in New World), lianas, vines, herbs or subshrubs, mostly perennial, often sending up annual shoots from a perennial rootstock, occasionally annual, and rarely strongly succulent (introduced in the Americas); latex usually white, but clear in non-native, succulent Ceropegieae. Leaves simple, entire, most commonly opposite but occasionally whorled, or reduced to a scale or spine (in introduced succulent genera).  Flowers actinomorphic, bisexual, 5-merous except for the paired carpels; corolla fused at least at the base, varying from rotate or shallowly campanulate to tubular or funnel-shaped, lobes twisted in bud and overlapping to the right, or valvate; coronal structures often present either on corolla (infrequently) or more commonly on gynostegium (formed from fusion of stamens and stylar head; stamens inserted at base of corolla, filaments fused into a tube, anthers with specialised marginal wings or guide rails; pollen aggregated into pollinia (one per locule, two per pollinarium), these bound by a waxy outer wall, pollinia attached to hard, brown or black, grooved corpusculum by translator arms, forming a pollinarium extracted from the flower as a unit; ovary superior, apocarpous with two carpels, these fused apically to form a stylar head with stigmatic surfaces on underside (secretions from the stylar head are formed into five discrete clip-like structures (corpuscula) which assist in pollen transport). Fruits paired dehiscent follicles (frequently single by abortion). Seeds generally flattened, ovate, mostly with a narrow marginal rim and a tuft of hairs at one end.

Notes on delimitation

  • The Asclepiadoideae is the most derived of five subfamilies now included within an expanded Apocynaceae.
  • Its extreme floral modifications, with complicated pollination syndromes and structures, often with corona elements in one or more whorls, and with pollen aggregated into pollinaria, led earlier workers to classify it as a separate family; but in reality the trend towards the complicated flowers of the Asclepiadoideae starts midway through Apocynaceae s.s., where the stylar head abutts closely underneath the cone of anthers, forming a functional gynostegium. From there on, progressive synorganisation of floral organs, accompanied by a shortening of the corolla tube, lead to greater control over pollen transfer, through the Apocynoideae and the Old-World subfamilies Periplocoideae and Secamonoideae, to the highly derived Asclepiadoideae.
  • The basal subfamily Rauvolfioideae is much more variable both vegetatively and florally than these more derived groups.

Distribution in the Neotropics

  • Throughout, but most diverse in seasonally dry habitats.

Distinguishing characters (always present)

  • Subfamily Asclepiadoideae can be characterised by the presence of a gynostegium in which the expanded stylar head is fused to the androecium by cell fusion, and where the narrow gap between the sclerified wings of adjacent anthers form five grooves.
  • The form of the pollinarium is also diagnostic for the subfamily, and indeed tribes within it can be determined by close observation of pollinarium structure (see below).
  • Pollen of one anther locule is aggregated into a pollinium encased by a waxy wall. This pollen mass is linked to another in an adjacent anther by a pair of translator arms and a single corpusculum. So two pollinia linked by translator arms to a single corpusculum form a pollinarium that can be extracted from the flower as a single unit. The corpusculum is usually black or dark brown, and sits at the top of the groove between adjacent anthers.

Other important characters

  • Other striking floral characters include a corona of one or more whorls. This can be derived from the corolla, or from the gynostegium, usually the stamens, but is occasionally absent.

Key differences from similar families

  • Subfamilies Apocynoideae and Rauvolfioideae (Apocynaceae in the narrow sense) lack the pollinarium - pollen is generally distributed as tetrads rather than pollinia. If a corona is present, then it is found only on the corolla. The stylar head and the cone of anthers are positioned part way up the corolla tube rather than at the base, and do not form a compound structure, the gynostegium. Fruit morphology is much more varied, particularly in the basal Rauvolfioideae, where seeds lack a coma of hairs, and the fruit is often an indehiscent berry.

Number of genera

c. 50 genera native to neotropics (largest genera listed first in each group, then alphabetical)

Marsdenieae

  • Marsdenia R.Br.

Ceropegieae

  • [Stapelia L., and possibly other genera - introduced]

Asclepiadeae: Asclepiadinae

  • Asclepias L.
  • [Calotropis R.Br., Gomphocarpus R.Br. - introduced]

Asclepiadeae: Cynanchineae

  • Cynanchum L. [note only c. 12 New World species belong here - most NW 'Cynanchum' are now referable to genera in the Metastelmatinae or Orthosiinae]

Asclepiadeae: MOG unplaced

  • Pentacyphus Schltr. [2 species - high Andean]
  • Diplolepis R.Br. [temperate S America]
  • Tassadia Decne.

Asclepiadeae: Gonolobinae (NW endemic subtribe)

  • Gonolobus Michx.
  • Matelea Aubl.
  • Dictyanthus Decne.
  • Fischeria DC.
  • Gyrostelma E.Fourn.
  • Hypolobus E.Fourn.
  • Labidostelma Schltr.
  • Macroscepis Kunth
  • Metalepis Griseb.
  • Pherotrichis Decne.
  • Polystemma Decne.
  • Prosthediscus Donn. Sm.
  • Rojasia Malme
  • Schubertia Mart.
  • Stelmagonum Baill.

Asclepiadeae: Metastelmatinae (NW endemic subtribe)

  • Ditassa R.Br.
  • Metastelma R.Br.
  • Ampelamus Raf.
  • Barjonia Decne.
  • Blepharodon Decne.
  • Cyathostelma E.Fourn.
  • Gonioanthela Malme
  • Hemipogon Decne.
  • Macroditassa Malme
  • Minaria T.U.P.Konno & Rapini
  • Nautonia Decne.
  • Nephradenia Decne.
  • Peplonia Decne.
  • Petalostelma E.Fourn.
  • Rhyssostelma Decne.

Asclepiadeae: Orthosiinae (NW endemic subtribe)

  • Jobinia E.Fourn.
  • Orthosia Decne.

Asclepiadeae: Oxypetalinae (NW endemic subtribe)

  • Oxypetalum R.Br.
  • Philibertia Kunth
  • Funastrum E.Fourn.
  • Araujia Brot.
  • Kerbera E.Fourn.
  • Morrenia Lindl.
  • Schistogyne Hook. & Arn.
  • Stenomeria Turcz.
  • Tweedia Hook. & Arn.
  • Widgrenia Malme

Useful tips for generic identification

  • If the plant is a stem-succulent with leaves reduced to scales, you are looking at an introduced Stapelia or allied genus in the Ceropegieae. The pollinaria will be held erect, and have a translucent germination zone.
  • If the pollinia are held erect, i.e. above the position of the corpusculum, and do not possess a translucent germination zone, then the tribe is Marsdenieae. Marsdenia is the only representative of this tribe in the New World - clue: if the anthers extend significantly beyond the position of the corpusculum, there will be space for the pollinia to be held erect. Marsdenia tends also to have a tubular corolla obscuring the gynostegium, and five erect staminal corona lobes. NB. Barjonia (Asclepiadeae) may have erect pollinia, but will never have a tubular corolla obscuring the gynostegium.
  • If the pollinia are pendant or horizontal relative to the position of the corpusculum, then the plant is in tribe Asclepiadeae - the vast majority of New-World asclepiads, and all the remaining NW genera.
  • Asclepias can be recognised easily by the combination of erect herb or subshrub AND a staminal corona formed of five concave fleshy organs with a tooth arising from the cavity of each lobe. Corolla and corona are often brightly coloured.
  • Now look at the pentagonal drum of stamens - the 'head' of the gynostegium. If this is flattened and subdiscoid, with very short anther wings relative to the diameter of the disc, the pollinia will be displaced laterally in relation to the corpusculum, and you are looking at Gonolobinae - the two big genera being Matelea and Gonolobus (see below for diagnostic features).
  • Of the remaining groups, the pollinaria of Oxypetalinae are generally large, and structurally complex, sometimes with teeth on the flattened translator arms. Flowers also tend to be large, and generally the stylar head extends beyond the staminal column as a well-developed appendage.
  • Genera of the Metastelmatinae and remaining unplaced groupings tend to have minute or small flowers.

Notable genera and distinguishing features

  • Asclepias - erect herbs or subshrubs; flowers generally brightly coloured, red, yellow or white, corona of 5 lobes on the back of the stamens, concave with a prominent tooth arising from the cavity of the lobe; follicles always single by abortion.
  • Stapelia - stem-succulent with large, foul-smelling, reddish or brownish flowers attracting flies (native to Africa).
  • Gonolobus - slender twiners; flowers rotate with exposed discoid head to gynostegium and laterally disposed pollinia, anthers with dorsal appendages; flowers commonly green; follicles with longitudinal wings.
  • Matelea - erect herbs or more usually slender twiners; flowers rotate with exposed discoid head to the gynostegium and laterally dispose pollinia, anthers laking dorsal appendages; flowers often smelly, green or reddish brown and fly-pollinated; follicles lacking longitudinal wings, but often with soft spiny processes.
  • Cynanchum - twiners; corona tubular, at least at the base, around the gynostegium.
  • Marsdenia - fairly robust twiners, often with large, thickish leaves, some species associated with inselbergs; corolla usually with well-developed tubular portion enclosing the gynostegium, corona of 5, generally fleshy erect lobes on back of stamens; pollinia erect.
  • Schubertia - twiner with large, urceolate white flowers.
  • Metastelma - twiners with small flowers; corolla lobes generally pubescent above, corona of 5 staminal lobes.
  • Ditassa - mostly twiners with small flowers; corolla lobes mostly not obviously pubescent, corona of five 'double' lobes - each lobe has a ligule on the inner face.
  • Minaria - ericoid shubshrubs with flowers similar to Ditassa (recently split from Ditassa).
  • Funastrum - mostly vigorous, free-flowering twiners often with their feet in seasonally flooded pools; stems semisucculent and photosynthetic (leaves often drop off); umbels of white or cream flowers, corona mosly with a tubular outer ring, and 5 fleshy lobes on the back of the anthers.
  • Oxypetalum - erect herbs or twiners; flowers moderate in size; mostly with conspicuous stylar head appendages (divided into 2 or more lobes); corona lobes arising from the corolla tube NOT on the back of the anthers; pollinaria large, and often with teeth on the translator arms; distribution centred on Brazil.
  • Philibertia - Andean version of Oxypetalum but with corona on back of stamens not on the corolla, or absent; stylar head appendages variable or absent.

Status

  • All native except those highlighted in list above [Stapelia, Calotropis, Gomphocarpus]. Most endemic to the New World, but Marsdenia is pan-tropical, and Asclepias is most diverse in North America and Africa, with just 12 species extending into central and South America.

General notes

 

Important literature

Endress, M.E. & Bruyns, P.V. (2000). A revised classification of the Apocynaceae s.l. The Botanical Review 66: 1 - 56.

Fontella Pereira, J. (1980). Estudos em Asclepiadaceae, XI. Chave para determinação dos gêneros de Asclepiadaceae brasileiras e mas cultivadas no Brasil. Bol. Mus. Bot. Mun. Curitiba 42: 1 - 6. [But note some genera recognised now in synonymy]

Goyder, D. J. (2003). A synopsis of Morrenia Lindl. (Apocynaceae subfam. Asclepiadoideae). Kew Bull. 58(3): 713 - 721.

Goyder, D. J. (2004). An amplified concept of Philibertia Kunth (Apocynaceae: Asclepiadoideae), with a synopsis of the genus. Kew Bull. 59: 415 - 451. [Includes key to genera of Oxypetalum group, and key to species of Philibertia]

Goyder, D. J. (2006). An overview of Asclepiad biogeography. In: S. A. Ghazanfar & H. Beentje (eds), Taxonomy and Ecology of African Plants and their conservation and sustainable use (Proceedings of the XVIIth AETFAT congress). pp. 205 - 214.

Heyne, C. O. (2000). A key to American asclepiad genera. Asklepios 79: 29 - 36. [Good in parts, but contains some errors particularly around Oxypetalum group]

Liede-Schumann, S., Rapini, A., Goyder, D.J. & Chase, M. (2005). Phylogenetics of the New World subtribes of Asclepiadeae (Apocynaceae - Asclepiadoideae): Metastelmatinae, Oxypetalinae and Gonolobinae. Syst. Bot. 30: 183 - 194.

Rapini, A., Goyder, D. J., Konno, T. U. P. & Farinaccio, M. A. (2005). Progress in asclepiad taxonomy: species numbers in Brazilian Asclepiadoideae (Apocynaceae) through time. Kew Bull. 60: 111 - 115.

Rapini, A., Mello-Silva, R. & Kawasaki, A. L. (2001). Asclepiadoideae (Apocynaceae) da Cadeia do Espinhaço de Minas Gerais, Brasil. Bol. Bot. Univ. São Paulo 19: 55 - 169.

How to cite

Goyder, D. (2009). Neotropical Apocynaceae (Asclepiadoideae). In: Milliken, W., Klitgård, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Apocynaceae_(Asclepiadoideae).htm.

Click images to enlarge


Araujia plumosa © John Wood, Darwin Initiative Project 161/11/015.



Asclepias barjoniifolia © John Wood, Darwin Initiative Project 161/11/015.



Asclepias barjoniifolia © John Wood, Darwin Initiative Project 161/11/015.



Asclepias curassavica © John Wood, Darwin Initiative Project 161/11/015.

 



Barjonia cymosa © John Wood, Darwin Initiative Project 161/11/015.



Blepharodon pictum © Daniela Zappi, RBG, Kew.



Cynanchum montevidense © Denise Sasaki, Projecto Flora Cristalino.



Ditassa eximia © Daniela Zappi, RBG, Kew.



Funastrum clausum © Denise Sasaki, Projecto Flora Cristalino.



Matelea boliviana © John Wood, Darwin Initiative Project 161/11/015.



Matelea friesii © John Wood, Darwin Initiative Project 161/11/015.



Matelea maritima © William Milliken, RBG, Kew.



Matelea maritima © William Milliken, RBG, Kew.



Metastelma parviflorum © John Wood, Darwin Initiative Project 161/11/015.



Minaria magisteriana © William Milliken, RBG, Kew.



Morrenia odorata © John Wood, Darwin Initiative Project 161/11/015.



Morrenia variegata © John Wood, Darwin Initiative Project 161/11/015.



Nephradenia linearis © Denise Sasaki, Projecto Flora Cristalino.



Oxypetalum warmingii © William Milliken, RBG, Kew.



Petalostelma sarcostemma © John Wood, Darwin Initiative Project 161/11/015.



Philibertia boliviensis © John Wood, Darwin Initiative Project 161/11/015.



Philibertia globiflora © John Wood, Darwin Initiative Project 161/11/015.



Philibertia lysimachioides © John Wood, Darwin Initiative Project 161/11/015.



Philibertia lysimachioides © John Wood, Darwin Initiative Project 161/11/015.



Philibertia picta © John Wood, Darwin Initiative Project 161/11/015.



Philibertia speciosa © John Wood, Darwin Initiative Project 161/11/015.



Philibertia zongoensis © John Wood, Darwin Initiative Project 161/11/015.



Schistogyne pentaseta © John Wood, Darwin Initiative Project 161/11/015.



Schubertia grandiflora  © John Wood, Darwin Initiative Project 161/11/015.