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A tumbleweed is a structural part of the above-ground anatomy of a number of species of plants, a diaspore that, once it is mature and dry, detaches from its root or stem, and tumbles away in the wind. In most such species the tumbleweed is in effect the entire plant apart from the root system, but in other plants a hollow fruit or an inflorescence might serve the function. Tumbleweed species occur most commonly in steppe and arid ecologies, where frequent wind and the open environment permit rolling without prohibitive obstruction. Apart from its propagules (that is, its seeds or spores), the tissues of the tumbleweed structure are dead; their death is functional because it is necessary for the structure to degrade gradually and fall apart so that the propagules can escape during the tumbling, or germinate after the tumbleweed has come to rest in a wet location. In the latter case, many species of tumbleweed open mechanically, releasing their seeds as they swell when they absorb water.〔 page 16〕 The tumbleweed diaspore disperses propagules, but the tumbleweed strategy is not limited to the seed plants; some species of spore-bearing Cryptogams such as ''Selaginella'' form tumbleweeds, and some fungi that resemble puffballs dry out, break free of their attachments and are similarly tumbled by the wind, dispersing spores as they go. ==Plants that form tumbleweeds== The tumbleweed dispersal strategies are unusual among plants; most species disperse their seeds by other mechanisms. Many tumbleweeds (though by no means all) are ruderal species, opportunistic agricultural weeds. Tumbleweeds have been recorded in the following plant families: * Amaranthaceae (now including Chenopodiaceae) * Amaryllidaceae * Apiaceae * Asphodelaceae * Asteraceae * Brassicaceae * Boraginaceae * Caryophyllaceae * Fabaceae * Lamiaceae * Poaceae In the family Amaranthaceae (s.l., including Chenopodiaceae), several annual species of the genus ''Kali'' are the most notorious tumbleweeds. They are thought to be native to Eurasia, but when their seeds entered North America in shipments of agricultural seeds, they became naturalized in large areas. They have been so successful that in the cinema genre of Westerns they have long been symbols of frontier areas. ''Salsola tragus'' (currently not valid synonym〔(【引用サイトリンク】title=Calflora: Salsola tragus )〕〔(【引用サイトリンク】title=Plants Profile for Salsola tragus (prickly Russian thistle) )〕 ''Kali tragus'') is the so-called "Russian thistle". It is an annual plant that breaks off at the stem base when it dies, and forms a tumbleweed, dispersing its seeds as the wind rolls it along. It is said to have arrived in the United States in shipments of flax seeds to South Dakota, perhaps about 1870. It now is a noxious weed throughout North America, dominating disturbed habitats such as roadsides, cultivated fields, eroded slopes, and arid regions with sparse vegetation. Though it is a troublesome weed, ''Salsola tragus'' also provides useful livestock forage on arid rangelands.〔(Salsola tragus Linnaeus ) in Vol. 4 Page 399, 400, 401, 402 (Flora of North America ), (eFloras.org ).〕 Other members of the Amaranthaceae (s.l.) that form tumbleweeds include ''Kochia'' species,〔 page 477〕 ''Cycloloma atriplicifolium'', and ''Corispermum hyssopifolium'',〔 which are called plains tumbleweed.〔(Chenopodiaceae, Standardized nomenclature ), Texas A&M University: Center for the Study of Digital Libraries.〕 ''Atriplex rosea'' is called the tumbling oracle or tumbling orach.〔(WILDLAND FIRE MANAGEMENT PLAN: SACRAMENTO NATIONAL WILDLIFE REFUGE COMPLEX ), U.S. Fish and Wildlife Service.〕〔(Atriplex rosea Linnaeus ), in Vol. 4 Page 326, 340, 358 (Flora of North America ), (eFloras.org ).〕 Among the Amaranthaceae (s.s.) that form tumbleweeds there are several species of ''Amaranthus'', such as ''Amaranthus albus'', native to Central America but invasive in Europe, Asia, and Australia; and ''Amaranthus graecizans'', native to Africa, but naturalized in North America. ''Amaranthus retroflexus'', which is indigenous to tropical North and South America, has become nearly cosmopolitan largely as a weed, but like many other species of ''Amaranthus'' it also is widely valued as animal forage and as human food, though it should be utilised with caution to avoid toxicity.〔Watt, John Mitchell; Breyer-Brandwijk, Maria Gerdina: The Medicinal and Poisonous Plants of Southern and Eastern Africa 2nd ed Pub. E & S Livingstone 1962〕 Several Southern African genera in the family Amaryllidaceae produce highly optimised tumbleweeds; their inflorescences are globular umbels with long, spoke-like pedicels, either effectively at ground level, or breaking off once the stems are dry. When the seeds are about ripe, the fruit remain attached to the peduncles, but the stem of the umbel detaches, permitting the globes to roll about in the wind. The light, open, globular structures form very effective tumbleweed diaspores, dropping their seeds usually within a few days as the follicles fail under the wear of rolling. The seeds are fleshy, short-lived, and germinate rapidly where they land. Being poisonous and distasteful, they are not attractive to candidate transport animals, so the rolling diaspore is a very effective dispersal strategy for such plants. Genera with this means of seed dispersal include Ammocharis, Boophone, Crossyne and Brunsvigia. Some species of the Apiaceae form tumbleweeds from their flower umbels, much as some Amaryllidaceae do.〔 In the Asteraceae, the knapweed ''Centaurea diffusa'' forms tumbleweeds. It is native to Eurasia and is naturalized in much of North America. Also in the Asteraceae, ''Lessingia glandulifera'', native to America, sometimes forms tumbleweeds; it grows on sandy soils in desert areas, chaparral, and open pine forests of the western United States.〔( Lessingia glandulifera ) in Vol. 20 Page 452, 454, 456 (Flora of North America ), (eFloras.org ).〕 In the Brassicaceae, ''Sisymbrium altissimum'', ''Crambe maritima'', ''Lepidium'', and a resurrection plant, ''Anastatica'' form tumbleweeds. In the Caryophyllaceae, the garden plant "baby's-breath" (''Gypsophila paniculata''), produces a dry inflorescence that forms tumbleweeds. In parts of central and western North America, it has become a common weed in many locations including hayfields and pastures.〔(Gypsophila paniculata Linnaeus ) in Vol. 5 (Flora of North America ), (eFloras.org ).〕 In the legume family (Fabaceae), ''Baptisia tinctoria'' and some species of ''Psoralea'' produce tumbleweeds. In ''Psoralea'' the tumbleweed detaches from the plant by abscission of the stem. In the Plantaginaceae, ''Plantago cretica'' forms tumbleweeds. Inflorescences that act as tumbling diaspores occur in some grasses, including ''Schedonnardus paniculatus'' and some species of ''Eragrostis'' and ''Aristida''. In these plants, the inflorescences break off and tumble in the wind instead of the whole plant, much as happens in some of the Apiaceae and Amaryllidaceae. The species of ''Spinifex'' from Southeast Asia are prominent examples of this dispersal adaptation. These grasses are often called tumble-grasses, including such species as ''Panicum capillare'' and ''Eragrostis pectinacea'' in the United States. In the Solanaceae, ''Solanum rostratum ''.〔 forms tumbleweeds. Wind dispersed fruits that tumble or roll on the ground, sometimes known as "tumble fruits", are rare. Some are technically achenes. Highly inflated indehiscent fruits that may facilitate tumbling include ''Alyssopsis'',〔 page 83〕 ''Coluteocarpus'',〔 ''Physoptychis'',〔 ''Sutherlandia'' and ''Physaria''.〔 Very similar in habit to ''Anastatica'', but practically unrelated, are the spore-bearing ''Selaginella lepidophylla'' (a lycopod) and earthstar mushroom family (Geastraceae). All of these curl into a ball when dry and uncurl when moistened. ''Bovista'', a genus of puffball, uses essentially the same dispersal strategy. 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「tumbleweed」の詳細全文を読む スポンサード リンク
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