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

Asteraceae Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...

. 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 a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

of over 50 species, including the physically different genera '' Dubautia'' and '' Wilkesia''. This grouping is often referred to as the silversword alliance. . 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 In horticulture, the term perennial ('' per-'' + '' -ennial'', "through the year") is used to differentiate a plant from shorter-lived annuals and biennials. It has thus been defined as a plant that lives more than 2 years. The term is also ...

, rosette-forming

shrub A shrub or 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 by their multiple ...

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

trichome 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 plant ...

s 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 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 ...

. The flowering stalks may be up to tall, and are composed of up to 600 capitulae. These

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

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 and require cross-pollination by

insects Insects (from Latin ') are 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 jointed ...

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

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

. 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 '' Dubautia''. 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. DNA analysis has revealed that silverswords form a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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. 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 only 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 foun ...

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

and occur only on the islands of

Maui Maui (; Hawaiian language, Hawaiian: ) is the second largest island in the Hawaiian archipelago, at 727.2 square miles (1,883 km2). It is the List of islands of the United States by area, 17th-largest in the United States. Maui is one of ...

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, quagm ...

, 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 Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

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 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 An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...

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, inv ...

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 an inventory of the global conservation status and extinction risk of biological ...

. Direct damage from humans and from

ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

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 an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

, but in 1989 plants were discovered that appear to be hybrids between it and the Haleakalā silversword. The hybrid is known as the Pu'u 'Alaea greensword.

Species

;Accepted species * '' Argyroxiphium caliginis'' C.N.Forbes – Eke silversword * '' Argyroxiphium grayanum'' ( Hillebr.) O. Deg. – greensword * '' Argyroxiphium × kai'' D.D.Keck ''(A. caliginis × A. grayanum)'' * '' Argyroxiphium kauense'' ( Rock & M.Neal) O.Deg. & I.Deg. – Mauna Loa or Kaū silversword * '' Argyroxiphium sandwicense'' DC. – silversword ** ''Argyroxiphium sandwicense'' subsp. ''sandwicense'' DC. – Mauna Kea silversword ** ''Argyroxiphium sandwicense'' subsp. ''macrocephalum'' ( A.Gray) Meyrat – Haleakalā silversword * '' Argyroxiphium virescens'' Hillebr. – East Maui greensword (

) ;Formerly included now in '' Wilkesia'' * ''A. gymnoxiphium – Wilkesia gymnoxiphium''

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 Plants

US Department of Agriculture plants profile
{{Taxonbar, from=Q2414693 Endemic flora of Hawaii Asteraceae genera