Adaptive Radiation of the Hawaiian Silversword Alliance

Flower and Fruit Diversity

Last Modification: February 14, 2004

Members of the sunflower family have aggregations of tiny flowers that mimic a single flower. Each flower mimic is called a head or capitulum. In the case of Argyroxiphium, the head comprises two kinds of flowers (i.e. it is heterogamous), petal-like female ray flowers around the periphery, and tiny tubular bisexual flowers in the center (disk flowers). The flowering stalk, sometimes called a capitulescence, may reach 1.5 m in length and bear up to 600 heads in the Haleakala silversword illustrated here.
In the Haleakala silversword each head has up to 40 peripheral ray flowers and 600 disk flowers. Flower color in the genus Argyroxiphium ranges from dark wine-red to yellowish or creamy white. The diameter of the heads including the rays ranges from about 1 to 6 cm. In A. caliginis the ray flowers are extremely reduced in size and number.
The heads in Wilkesia are about 2 to 3.5 cm in diameter, lack ray flowers (i.e. are homogamous), and are cream-colored to yellowish. The flowering stalk may reach 1 m in length and bear 350 or more heads, each with 50 to 200 or more disk flowers.
As is the case in Wilkesia, the flowering heads of Dubautia lack ray flowers. They are also smaller, the largest heads being found in Dubautia arborea (l), with 45 or fewer disk flowers. The number of heads on each flower stalk or capitulescence varies from 3 to several hundred among the species of the genus. Whereas in some species of Argyroxiphium and Wilkesia an individual plant bears only one flowering stalk and dies when fruiting is complete (monocarpy), individuals of species of Dubautia potentially bear many flowering stalks each season over many seasons (polycarpy).
Flower colors in Dubautia range from deep orange-yellow in several species to nearly pure white, e.g., D. raillardioides (l) and Dubautia scabra. Reddish hues may be seen in a few instances, especially as flowers age.
The smallest flowering heads of any species in the silversword alliance are found in Dubautia pauciflorula, where each head consists of only 2 to 4 tiny disk flowers.
In addition to the two kinds of flowers in the heads of Argyroxiphium, there are two kinds of bracts. An outermost involucral bract (sometimes called a phyllary) envelops the fruit (achene or cypsella) of each of the ray flowers at the extreme periphery of the flowering head. Interior to these are receptacular bracts (sometimes called chaff). In Argyroxiphium, Wilkesia, and some species of Dubautia, these receptacular bracts are restricted to the perimeter of the disk and are laterally coalesced to form a cylinder that encircles the tubular disk flowers.
The flowering heads of Wilkesia and Dubautia lack ray flowers and the involucral bracts normally associated with them. Moreover, the distribution of bracts varies in Dubautia. Some species such as Dubautia reticulata (l) have a cylinder of laterally fused receptacular bracts enclosing the disk flowers while others, e.g., Dubautia paleata (r), have a separate receptacular bract associated with each disk flower in the head. Intermediate types also occur.
The achenes of Argyroxiphium are the largest in the alliance, ranging from 4-15 mm in length, depending on the species. The modified calyx or pappus at the summit of the fruit varies from a minute rim such as seen on the ray achene (r) to a series of asymmetrically developed scales as evident on the disk achene (l).
Achenes of Wilkesia range from 4.5-7 mm in length and are crowned by several lanceolate pappus segments.
Dubautia has the smallest achenes in the alliance, ranging from 2-6.5 mm in length. The pappus is variable, ranging from pectinate bristles that are longer than the achene body (l) to erose-margined scales that are much shorter than the achene (r).

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