''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 In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

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''. 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

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...

s. 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 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 are ...

individuals with a single rosette, this ends in the death of the plant, though some individuals are polycarpic. The flowering stalks may be up to tall, and are composed of up to 600 capitulae. These

flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...

heads range in diameter from to and consist of a ring of

pistillate Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils'' ...

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, birds, ...

by insects, many plants must flower at the same time in relatively close proximity to set

seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...

. 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 (biology), 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 Silversword alliance, silver ...

. DNA analysis has revealed that silverswords form a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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 Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

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

. 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 Wetland#Types, types of wetlands. Other names for bogs include mire, mosses, quagmire, ...

, alpine shrublands, or wet shrublands. ''A. sandwichense'' is able to grow at high altitudes between and on cinder and

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

with relatively little rainfall. The Haleakalā silversword (''A sandwicense'' ssp. ''macrocephalum'') is constrained to Haleakalā on Maui while the Mauna Kea silversword (''A. sandwicense'' ssp. ''sandwicense'') is specific to

Mauna Kea Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii and second-highest peak of an island on Earth. The peak is ...

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 An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and inva ...

in the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...

. 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, cam ...

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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

, 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 (biology), family Asteraceae. The silversword in general is referred to as ''āhinahina'' in Hawaiian ...

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, fr ...

**''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 exc ...

) Meyrat –

*'' Argyroxiphium virescens'' Hillebr. – East Maui greensword (

) ;Formerly included now in '' Wilkesia'' *''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 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