|
''Eurybia divaricata'' (formerly ''Aster divaricatus''), commonly known as the white wood aster, is an herbaceous plant native to eastern North America. It occurs in the eastern United States, primarily in the Appalachian mountains, though it is also present in southeastern Canada, but only in about 25 populations in the provinces of Ontario and Quebec. In the U.S. it is abundant and common, but in Canada it is considered threatened due to its restricted distribution. It can be found in dry open woods as well as along wood-edges and clearings. The species is distinguished by its flower heads that have yellow centers and white rays that are arranged in flat-topped corymbiform arrays, emerging in the late summer through fall. Other distinguishing characteristics include its serpentine stems and sharply serrated narrow heart-shaped leaves. The white wood aster is sometimes used in cultivation in both North America and Europe due to it being quite tough and for its showy flowers. ==Description== ''Eurybia divaricata'' is a late summer to fall-flowering herbaceous perennial, typically growing to heights between 30 and 90 cm, though some specimens may be up to 1.2 metres (4 feet) tall. The plant emerges each year from rhizomes and forms dense colonies of clones that lack sterile rosettes. The rhizomes are branched, elongated and become woody with age. One simple erect stem is present per plant. It is flexible and nearly hairless to finely hairy near the base, though densely covered with fine hair towards the extremities. It is very similar to and often confused with ''E. chlorolepis'', ''E. schreberi'' and ''Symphyotrichum cordifolium'', though ''E. schreberi'' differs in having wider leaves with more teeth, while ''E. chlorolepis'' has more rays, longer involucres, and only occurs in the southern U.S. from Virginia to Georgia. Three types of leaves with differing morphology are present: cauline leaves, or those that appear on the stem from the middle of the plant upwards; basal leaves, or those that are present at the base of the plant; and distal leaves, which are those found on the extremities of the plant. All types are thin and sharply serrated with 6 to 15 pointed teeth per side. They are ciliate, meaning they have small hairy projections emerging from the margins of the leaf, while the apices, or tips of the leaves, are acuminate, meaning they taper to a point. The adaxial (i.e. upper) surfaces of the leaves are nearly hairless or sparsely hairy, while the abaxial (i.e. lower) surfaces are sparsely hairy with the veins being more villous, or covered in shaggy hairs.〔 The basal leaves are ovate, or egg-shaped, with bases that are cordate, or heart-shaped. The blades measure 1.9 to 6.5 cm in length by 1.7 to 6 cm (0.7-2.4 in) in width and have petioles ranging in length from 2 to 7 cm. They wither when the plant flowers. The cauline leaves have petioles measuring 2.5 to 7 mm (0.1-0.3 in) long that are often winged. Their blades are also ovate, though the bases may be cordate to rounded. They measure 2 to 20 cm long by 1 to 10 cm (0.4-4.0 in) wide, making them often much longer than the basal leaves. The distal leaves are typically sessile, meaning that no petiole is present, though they are sometimes subpetiolate, meaning a very short petiole is present. The blades are ovate to lanceolate, meaning lance-shaped, with rounded bases and are 0.5 to 2 cm (0.2-0.8 in) long by 0.1 to 0.8 cm wide.(0.04-0.32 in) 〔 The capitula, or flower heads, are arranged in relatively flat-topped corymbiform arrays. The capitula number anywhere from 4 to 50 and up to 100 or more in exceptional cases. The peduncles, i.e. the flower stalks, are up to 1.5 cm (0.6 in) in length and are densely covered with non-glandular hairs. Bracts, modified leaves that appear at the axil of a peduncle, are typically absent, though in some cases up to two are present. The involucres, which are the whorls of small, scale-like modified leaves that appear at the base of the capitulum, are in between cylindric and campanulate (i.e. bell-shaped) in shape and measure 4.2 to 6 mm (0.17 to 0.24 in) long, making them much shorter than the pappi.〔 The phyllaries, which are the small leaves that make up the involucre, number from 25 to 30 and are arranged in 4 to 5 series. The inner phyllaries are between linear and lanceolate in shape with a purplish colouration towards the apices, while the outer ones are more oblong. All of the phyllaries are strongly unequal with hardened bases, margins that are somewhat scarious (i.e. thin, membranous and dry) as well as fimbriate-ciliate, meaning fringed with hair at the margins. Their apices are rounded to acute in shape, while the surfaces are sparsely haired, though sometimes sparsely covered in stipular glands. Their chlorophyllous zones, a darker green zone where chlorophyll is concentrated, appear on the upper half of the outer phyllaries, to the upper third or along the outer midveins of the inner phyllaries. The outer phyllaries typically measure 0.7 to 1.5 mm wide (0.03-0.06 in) with the lengths rarely exceeding 2.5 times the width.〔 As with most members of the composite family, the actual flowers appear in two different froms: as ray florets, which have strap-like appendages that look like petals and project around the outside of the capitulum, and as disc florets, which appear at the center of the flower head and are very small. The ray florets number between 5 and 10, though as many as 12 may be present. Their straps are white and measure 6 to 12 mm long by 1.5 to 2.2 mm wide. The disc florets number from 12 to 19 and up to 25 and have yellow corollas (i.e. petals, though they are fused into a tube) that are 4.1 to 4.8 and exceptionally 5.5 mm long. Their corollas are abruptly ampliate, or enlarged, with tubes that are longer than their campanulate throats. The tubes measure 2.3 to 2.6 mm while the throats are typically only 0.9 to 1.2 mm long. The lobes, i.e. the friges of the throat, are reflexed and lanceolate in shape, measuring 0.7 to 1.4 mm.〔 The fruit are cypselae, a type of achene, which are brown in colour, slightly compressed and are between cylindric and obovoid, or inversely egg-shaped. They are between 2.6 and 3.8 mm (0.1-0.15 in) in length and sparsely strigillose, or set with stiff bristly hairs, with 7 to 10 ribs, which themselves are tan to stramineous (i.e. straw-coloured). The pappi, which are modified sepals, are made up of reddish to cream-coloured bristles that are 3.7 to 5 mm (0.15-0.20 in) long, making them equal to or longer than the disc corollas in length. The bristles are fine and barbellulate, or barb-like, though they may be sometimes more or less clavate, or club-shaped, towards their apices.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eurybia divaricata」の詳細全文を読む スポンサード リンク
|