''Eupatorium rotundifolium'', commonly called roundleaf thoroughwort, is a North American species of plant in the family

Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae ...

. It native to the eastern and central

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

, in all the coastal states from

Maine Maine () is a U.S. state, state in the New England and Northeastern United States, Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Provinces and territories of Canad ...

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

, and inland as far as

Missouri Missouri is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it is bordered by eight states (tied for the most with Tennessee ...

and the

Ohio Valley The Ohio River is a long river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing southwesterly from western Pennsylvania to its mouth on the Mississippi River at the southern tip of Illino ...

. It is found in low, moist habitats such as wet savannas and bogs.Flora of North America, Eupatorium rotundifolium Linnaeus, 1753. Roundleaf thoroughwort
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Description

The stems up to 100 cm (40 inches) tall and are produced from short rhizomes. The

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed ...

s are composed of a large number of small white flower heads, each with 5

disc floret The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae we ...

s but no

ray florets The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae we ...

. Plants can be highly variable due to hybridization. Image:Eupatorium_rotundifolium_NRCS-2.jpg, Detail of stem and leaves. (Photo by Robert H. Mohlenbrock)

Taxonomy

Three varieties of ''Eupatorium rotundifolium'' are recognized. They are: *''Eupatorium rotundifolium'' var. ''ovatum'' (Bigelow) Torr. *''Eupatorium rotundifolium'' var. ''rotundifolium'' *''Eupatorium rotundifolium'' var. ''scabridum'' (Elliott) A.Gray The plants known as ''Eupatorium rotundifolium'' var. ''saundersii'' have often been treated as a variety of ''E. rotundifolium''. They can be distinguished based on

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

, and molecular evidence also suggests that these plants may be different enough from ''E. rotundifolium'' to recognize them as a species, ''Eupatorium pilosum''.

Hybridization

As is common in ''

Eupatorium ''Eupatorium'' is a genus of flowering plants in the family Asteraceae, containing from 36 to 60 species depending on the classification system. Most are herbaceous perennials growing to tall. A few are shrubs. The genus is native to temperate ...

'', ''E. rotundifolium'' can form hybrids with other species in the genus. In particular, ''

Eupatorium godfreyanum ''Eupatorium godfreyanum'', commonly called Godfrey’s thoroughwort, is a North American species of plants in the family Asteraceae. It is found in the east-central United States, primarily from Pennsylvania to North Carolina, with a few isolate ...

'' is a hybrid of ''E. rotundifolium'' and ''

Eupatorium sessilifolium ''Eupatorium sessilifolium'', commonly called upland boneset or sessile-leaved boneset, is a North American plant species in the family Asteraceae. It is native to the eastern and central United States, found from Maine south to North Carolina ...

''.

Chemical constituents

''Eupatorium rotundifolium'' contains

sesquiterpene lactone Sesquiterpene lactones (SLs) are a class of sesquiterpenoids that contain a lactone ring. They are most often found in plants of the family Asteraceae (daisies, asters). Other plant families with SLs are Umbelliferae (celery, parsley, carrots ...

s of the

guaianolide In organic chemistry, a guaianolide is a type of sesquiterpene lactone consisting of a gamma-lactone and either a cyclopentane or cyclopentene, both fused to a central cycloheptane or cycloheptene structure. There are two subclasses, structural ...

type including euparotin acetate and eupachlorin acetate, both of which inhibit tumor growth ''

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and ...

'' when isolated from the plant.

References

rotundifolium Flora of the United States Plants described in 1753 Taxa named by Carl Linnaeus {{Eupatorieae-stub