Alawa's sandbox
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The Puu Kukui greensword (Argyroxiphium grayanum) is a species of flowering plant in the sunflower family, Asteraceae and a constituent species of what is called the silversword alliance, a group of about 30 species which are diverse in morphology and habitat but are genetically exceedingly closely-related. Few species offer greater support to the observation by famed evolutionary biologist Stephen Jay Gould that islands are nature's evolutionary laboratories.[2] The silversword alliance provides a convincing natural case study in evolution by adaptive radiation, with the greensword representing one extreme of the genus' plasticity. It is thought that the ancestor of Argyroxiphium was a drought-adapted plant capable of storing water as a gel in leaf structures which are normally air pockets in other plants. Many Argyroxiphium live in harsh alpine desert-like conditions of heat, sun, wind, and aridity where plants without this unusual physiology cannot survive. However, A. grayanum has adapted to very different conditions— excessive moisture, lack of regular sunlight, and cool temperatures.[3]
Description edit
A. grayanum is a perennial plant endemic to Hawaiʻi, occurring only on the island of Maui in an extremely limited habitat, two cool rainforest bogs which are almost constantly mist-shrouded, with elevations ranging from about 1,200 to 2,050 m. The sites receive about 150->1,000 cm precipitation per year.[4] The Puu Kukui Bog gives its name to the common term for the plant, the Puu Kukui greensword. This same bog is also home to a closely-related species, A. caliginis, or ʻEke Silversword which is sometimes referred to as the Puu Kukui silversword.[5] Despite their close relationship and shared habitat, the two species differ in several ways beyond the coloring of their spear-shaped leaves, with silversword possessing a distinctive sheen. Members of the Argyroxiphium genus generally produce one inflorescence, following which the plant dies. Neither A. caligini, the silversword, nor its close-living greensword cousin A. grayanum follow this pattern in a strict sense. A. caligini flowers infrequently and, while the flowering stalk will wither, it reproduces by way of runners or prostate stems which root and spread. A. grayanum flowers repeatedly without dying back, producing light yellow flowers in smaller clusters than most members of its genus. Its flowers tend to be horizontal or pendant, an apparent adaptation to its extremely wet environment. It does not produce runners.[6]
Other greensword species edit
Greensword species include the A. virescens, extinct.[7] Interestingly, the virascens greensword was found in proximity to the Haleakala silversword with which it shared a similar growth form.[8]
Adaptive radiation and the silversword alliance edit
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 (Raillardiopsis murii) found in western North America.[9] That several spontaneous hybrids have been observed supports this hypothesis.[10] If true, the adaptive radiation of the tarweed/silversword/greensword into extremely diverse morphologies and habitats is an extraordinary case history of evolution.[11]
References edit
- ^ "Genus Argyroxiphium". Taxonomy. UniProt. Retrieved 2009-12-06.
- ^ Gould, S.J., Eight Little Piggies, Reflections in Natural History. "Unenchanted Evenings" WW Norton & Co. NY. 1993 (page 25)
- ^ Carlquist, Sherwin. Hawaii A Natural History. Pacific Tropical Botanical Garden, Lawai 1980. ISBN 0-915809-19-2 (page 264)
- ^ "Hawaiian Native Plant Genera - Asteraceae". University of Hawaii Botany Department. Retrieved 08 December 2009.
{{cite web}}: Check date values in:|accessdate=(help) - ^ Carlquist, Sherwin. Hawaii A Natural History. Pacific Tropical Botanical Garden, Lawai 1980. ISBN 0-915809-19-2 (page 264)
- ^ Carlquist, Sherwin. Hawaii A Natural History. Pacific Tropical Botanical Garden, Lawai 1980. ISBN 0-915809-19-2 (page 266)
- ^ Bruegmann, M.M. & Caraway, V. 2003. Argyroxiphium virescens. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on 08 December 2009.
- ^ Carlquist, Sherwin. Hawaii A Natural History. Pacific Tropical Botanical Garden, Lawai 1980. ISBN 0-915809-19-2 (page 264)
- ^ "Hawaiian Native Plant Genera - Asteraceae". University of Hawaii Botany Department. Retrieved 08 December 2009.
{{cite web}}: Check date values in:|accessdate=(help) - ^ "Natural and Artificial Hybrids in the Hawaiian Silversword Alliance". University of Hawaii Botany Department. Retrieved 08 December 2009.
{{cite web}}: Check date values in:|accessdate=(help) - ^ "Adaptive Radiation and Hybridization in the Hawaiian Silversword Alliance". University of Hawaii Botany Department. Retrieved 08 December 2009.
{{cite web}}: Check date values in:|accessdate=(help)
[[Category:Argyroxiphium|grayanum]] [[Category:Endemic flora of Hawaii]] [[Category:Vulnerable plants]]