''Argyroxiphium'' is a small genus of plants in the family

Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...

. Its members are known by the common names silversword or greensword due to their long, narrow leaves and the silvery hairs on some species. The silverswords belong to a larger radiation of over 50 species, including the physically different genera ''

Dubautia ''Dubautia'' is a genus of flowering plant in the family Asteraceae. The genus was named after Joseph Eugène DuBaut (1796-1832), an officer in the French Navy who participated in Freycinet's expedition. The entire genus is endemic to Hawaii. I ...

'' and ''

Wilkesia ''Wilkesia'' is a genus of Hawaiian plants in the tribe Madieae within the family Asteraceae. It contains two perennials, both of which are endemic to the island of Kauai in Hawaii. ''Wilkesia'' is a component of the silversword alliance and ...

''. This grouping is often referred to as the

silversword alliance The silversword alliance, also known as the tarweeds, refers to an adaptive radiation of around 30 species in the composite or sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands. Fo ...

. . Baldwin. Age and Rate of Diversification of the Hawaiian Silversword Alliance (Compositae). 1998./ref> Botanist P. H. Raven referred to this radiation as "the best example of adaptive radiation in plants".

Description

Species in ''Argyroxiphium'' are

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

, rosette-forming shrubs. They may consist of a single large rosette or a collection of several rosettes. In all ''Argyroxiphium'' species, long, narrow leaves contain interstitial gels hypothesized to function as water storage. For some species, leaves are covered with

trichomes Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plan ...

that provide protection from frost and create the plants' signature silver sheen. A silversword rosette grows for at least five years before flowering, though some species grow for up to 90 years before initiating the bolting process. For

monocarpic Monocarpic plants are those that flower and set seeds only once, and then die. The term is derived from Greek (''mono'', "single" + ''karpos'', "fruit" or "grain"), and was first used by Alphonse de Candolle. Other terms with the same meaning ar ...

individuals with a single rosette, this ends in the death of the plant, though some individuals are

polycarpic Polycarpic plants are those that flower and set seeds many times before dying. A term of identical meaning is pleonanthic and iteroparous. Polycarpic plants are able to reproduce multiple times due to at least some portion of its meristems being a ...

. The flowering stalks may be up to tall, and are composed of up to 600 capitulae. These flower heads range in diameter from to and consist of a ring of pistillate ray florets around 30 to 600 disk florets. The corollae vary in color from wine red to yellow or white. Because they are

self-incompatible Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals ...

and require

cross-pollination Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, bird ...

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of ...

, many plants must flower at the same time in relatively close proximity to set seed. A significant population must exist for enough individuals to flower simultaneously and allow pollination to occur. The single-seeded fruits are usually dispersed by wind. Despite their different appearances, silverswords are very closely related to the genus ''

''. Sympatric species in ''Argyroxiphium'' and ''Dubautia'' often naturally produce fertile hybrids that run the gamut of morphological characteristics from the two genera. Together, ''Argyroxiphium'', ''Dubautia'', and ''Wilkesia'' make up the silversword alliance.

Evolution

The evolutionary roots of ''Argyroxiphium'' are the tarweeds in subtribe

Madiinae Madieae is a tribe of flowering plants in the family Asteraceae. It is sometimes considered a subtribe of Heliantheae. Notable species include the tarweeds of the Western United States as well as the silverswords of Hawaii. Subtribes and g ...

. DNA analysis has revealed that silverswords form a clade within the Californian tarweed lineage. The relation is also physically evident—silversword capitulae and the flowers of Californian tarweeds both include sticky bracts that provide adhesion to birds for

seed dispersal In Spermatophyte plants, seed dispersal is the movement, spread or transport of seeds away from the parent plant. Plants have limited mobility and rely upon a variety of dispersal vectors to transport their seeds, including both abiotic vectors, ...

. It is hypothesized that an individual plant from the Californian tarweeds was spread first to Kauai, then spread to the other islands and developed into the silversword alliance.

Distribution

Silverswords are endemic to Hawaii and occur only on the islands of

Maui The island of Maui (; Hawaiian: ) is the second-largest of the islands of the state of Hawaii at 727.2 square miles (1,883 km2) and is the 17th largest island in the United States. Maui is the largest of Maui County's four islands, which ...

and Hawaii. They grow primarily over above sea level in

bogs A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; a ...

, alpine shrublands, or wet shrublands. ''A. sandwichense'' is able to grow at high altitudes between and on cinder and lava with relatively little rainfall. The Haleakalā silversword (''A sandwicense'' ssp. ''macrocephalum'') is constrained to

Haleakalā Haleakalā (; Hawaiian: ), or the East Maui Volcano, is a massive shield volcano that forms more than 75% of the Hawaiian Island of Maui. The western 25% of the island is formed by another volcano, Mauna Kahalawai, also referred to as the West ...

on Maui while the Mauna Kea silversword (''A. sandwicense'' ssp. ''sandwicense'') is specific to Mauna Kea on Hawaii. Each of the other species is found primarily at lower altitudes with much higher annual rainfall.

Conservation

The Mauna Loa or Kaū silversword (''A. kauense'') has been classified as critically endangered and the Mauna Kea and Halekalā silverswords (''A. sandwichense'') have been classified as endangered in the IUCN Red List. Direct damage from humans and from

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, c ...

browsing have significantly damaged silversword populations, but dedicated management efforts have resulted in successful conservation of some species. In particular, the Haleakalā silversword population reached a low of approximately 4,000 plants in the 1920s, but rebounded to over 6,500 individuals by 1970. On the other hand, the Mauna Kea silversword population was composed of approximately 50 naturally occurring individuals and 500 outplanted individuals in 1999. The East Maui greensword (''A. virescens'') is apparently extinct, but in 1989 plants were discovered that appear to be hybrids between it and the

Haleakalā silversword ''Argyroxiphium sandwicense'' subsp. ''macrocephalum'', the east Maui silversword or Haleakala silversword, is a rare plant, part of the family Asteraceae. The silversword in general is referred to as ''āhinahina'' in Hawaiian (literally, "ver ...

Species

;Accepted species *'' Argyroxiphium caliginis'' C.N.Forbes – Eke silversword *''

Argyroxiphium grayanum ''Argyroxiphium grayanum'', commonly known as the greensword, is a species of flowering plant in the family Asteraceae, and a member of the silversword alliance, a group of over 50 species which are diverse in morphology and habitat but are gene ...

'' ( Hillebr.) O. Deg. – greensword *'' Argyroxiphium × kai'' D.D.Keck ''(A. caliginis × A. grayanum)'' *'' Argyroxiphium kauense'' (

Rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...

& M.Neal) O.Deg. & I.Deg. – Mauna Loa or Kaū silversword *'' Argyroxiphium sandwicense''

DC. Augustin Pyramus (or Pyrame) de Candolle (, , ; 4 February 17789 September 1841) was a Swiss botanist. René Louiche Desfontaines launched de Candolle's botanical career by recommending him at a herbarium. Within a couple of years de Candolle ...

– silversword **''Argyroxiphium sandwicense'' subsp. ''sandwicense'' DC. –

Mauna Kea silversword ''Argyroxiphium sandwicense'' subsp. ''sandwicense'', the Mauna Kea silversword, is a highly endangered flowering plant endemic to the island of Hawaiʻi (''Big Island'') of Hawaii. It is the "crown jewel" of the volcanic mountain Mauna Kea, ...

**''Argyroxiphium sandwicense'' subsp. ''macrocephalum'' (

A.Gray Asa Gray (November 18, 1810 – January 30, 1888) is considered the most important American botanist of the 19th century. His ''Darwiniana'' was considered an important explanation of how religion and science were not necessarily mutually excl ...

) Meyrat –

*'' Argyroxiphium virescens'' Hillebr. – East Maui greensword ( extinct) ;Formerly included now in ''

'' *''A. gymnoxiphium -

Wilkesia gymnoxiphium ''Wilkesia gymnoxiphium'' ( Hawaiian iliau), is a species of flowering plant in the family Asteraceae that is endemic to the island of Kauai in Hawaii. It is classified as endangered on the IUCN Red List. ''Wilkesia'' is one of three genera, with ...

Gallery

File:Starr 980630-1521 Argyroxiphium grayanum.jpg, Greensword File:Argyroxiphium grayanum Puu Kukui.jpg, Greensword,

Puu Kukui Puu or Pu'u is a Hawaiian word for any protuberance, from a pimple to a hill, and can refer to: * Puu Kukui, mountain peak in Hawaii * Pu'u Huluhulu (Hawaii Route 200) * Pu'u 'Ō'ō (Puu Oo), volcanic cone in Hawaii * Setsuna Meioh (Sailor Pluto), ...

bog File:Silversword Haleakala.jpg, Haleakalā silversword File:Haleakala silversword.jpg, Silversword in Haleakalā Crater File:Dubautia silversword hybrid.jpg, ''Dubautia''-silversword hybrid in Haleakalā Crater File:Starr 011205-0093 Argyroxiphium sandwicense subsp. sandwicense.jpg, Mauna Kea silversword File:Mauna Kea silversword2.jpg, Mauna Kea silversword File:Mauna Kea silversword3.jpg, Mauna Kea silversword flowering File:MaunaKeaSilversword.JPG, Mauna Kea silversword File:Kahuku silversword habitat.jpg, Mauna Loa silversword habitat, Kahuku File:Kahuku silversword1.jpg, Mauna Loa silversword, multiple rosettes File:Kahuku silversword2.jpg, Mauna Loa silversword, single rosette File:Silversword (Healakala) close up.jpg, Haleakalā silversword (close-up) File:Mauna Loa silversword flowering.jpg, Mauna Loa silversword flowering

References

External links

* *
Kaʻahele Hawaiʻi Native PlantsUS Department of Agriculture plants profile
{{Taxonbar, from=Q2414693 Endemic flora of Hawaii Asteraceae genera