''Argyroxiphium grayanum'', commonly known as the greensword, is a species of

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

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 ...

, and a member of 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 ...

, a group of over 50 species which are diverse in morphology and habitat but are genetically closely related. The silversword alliance provides a convincing natural case study in evolution by

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

, with the greensword representing one extreme of the genus' plasticity. Some ''Argyroxiphium'', including the well-known Haleakala and

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 ...

s, live in harsh alpine desert-like conditions of heat, sun, wind, and aridity, and are drought-adapted plants capable of storing water as a gel in leaf structures which are normally air pockets in other plants. However, ''A. grayanum'' is a bog plant adapted to very different conditions – excessive moisture, lack of regular sunlight, and cool temperatures, and its leaves are non-succulent like those of the related genus ''

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 ...

''.

Description

''Argyroxiphium grayanum'' is a

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 ...

plant

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 ...

to the island 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 ...

in Hawaii. Its growth form is typically a low shrub up to 2 m high, erect, with an erect single-stemmed monocarpic rosette shape, though in the interior of

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, ...

it typically grows as a dwarf shrub under 30 cm high. It has green, 5-11-nerved, narrowly elliptic-ligulate leaves which are broadest above the middle.Wagner, W. L., D. R. Herbst, and S. H. Sohmer (1990). Manual of the Flowering Plants of Hawaii. Honolulu: University of Hawaii Press. It occurs only in and around montane cloud forest bogs at elevations ranging from about 1,200 to 2,050 m. The sites receive from about 300 to over 1,000 cm precipitation per year. It is most abundant along the upper rim of Kīpahulu Valley on East Maui and near the summit of

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), ''Sa ...

on West Maui. The latter region is also home to a related species, the Eke silversword ('' A. caliginis''). Despite their close relationship and shared habitat, the two species differ in several ways beyond the coloring of their lance-shaped leaves, with silversword possessing a distinctive sheen. Most ''Argyroxiphium'' species generally produce one inflorescence, after which the plant dies. Neither ''A. grayanum'' nor the

sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

Eke silversword (''A. caliginis'') follow this pattern in a strict sense. Both species flower infrequently compared to the mass flowerings of the Haleakala silversword, and produce multiple branches such that only some rosettes of a given plant die back in any given year. ''A. caliginis'' additionally reproduces by way of runners or prostrate stems which root and spread.

Other greensword species

Another greensword species '' A. virescens'', was formerly found on East Maui only, but is now apparently extinct.

Adaptive radiation and the silversword alliance

Starr 980630-1521 Argyroxiphium grayanum

Based on biosystematics and molecular studies, all of the silversword alliance species are thought to have evolved from a single ancestor, related to the tarweed (''

Carlquistia muirii ''Carlquistia'' is a rare North American genus of flowering plants in the family Asteraceae containing the single species ''Carlquistia muirii''. Formerly named ''Raillardiopsis muirii'', the plant was reexamined in the 1990s and moved to a new ...

'') found in western North America. That several spontaneous hybrids have been observed supports this hypothesis. If true, the adaptive radiation of the tarweed/silversword/greensword into extremely diverse morphologies and habitats is an extraordinary case history of evolution.

References

External links

* * {{Taxonbar, from=Q4790023 grayanum Endemic flora of Hawaii Biota of Maui