Argyroxiphium DC.

 

Argyroxiphium is a small genus of plants in the family Asteraceae. 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 of over 50 species, including the physically different genera Dubautia and Wilkesia. This grouping is often referred to as the silversword alliance. Botanist P. H. Raven referred to this radiation as "the best example of adaptive radiation in plants".

Description

Species in Argyroxiphium are perennial, rosette-forming shrubs. 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 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 1.5 metres (4.9 feet) tall, and are composed of up to 600 capitulae. These flower heads range in diameter from 1 cm (0.39 in) to 6 cm (2.4 in) 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, many plants must flower at the same time in relatively close proximity to set seed. 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.

Argyroxiphium - Wikipedia

 

 

 

 

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