''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 (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

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 ...

, and a member of the silversword alliance, 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 silverswords, 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 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 musk ...

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''.

Description

''Argyroxiphium grayanum'' is a

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 ...

plant

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 ...

to the island 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 ...

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, quagm ...

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) *Puʻu ʻŌʻō Puu Ōō (also spelled Pu‘u‘ō‘ō, and often written P ...

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 closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...

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'') 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