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Genus Plagius in Tribe Anthemideae

In botanical taxonomy, a genus (plural genera) is a rank used to group closely related species within a family. In the hierarchy, genus sits below family and above species.

Genera are defined by shared morphological, anatomical, and genetic characteristics (for example, features of flowers, fruits, seeds, or leaves) that indicate a close evolutionary relationship among the species they contain.

Each genus can include one or more species. Examples include Rosa (roses) and Solanum (nightshades, including tomato and eggplant).

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Genus Description

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Plagius (L'Hér. ex DC.) is a small genus of Asteraceae, tribe Anthemideae, currently comprising about four accepted species (POWO, 2024; WFO, 2024). Its representatives occur across the Mediterranean Basin, from the Iberian Peninsula to the Levant and North Africa, inhabiting dry, open habitats such as rocky slopes, garrigue and grasslands. The type species is not universally fixed in recent literature, and most treatments simply use the generic name without a designated lectotype.

Plants are herbaceous perennials forming basal rosettes of deeply lobed, densely tomentose leaves; stipules are absent. Each rosette gives rise to one or several erect stems that bear solitary, terminal capitula. The heads are radiate with a series of ligulate ray florets surrounding many tubular disc florets; corollas are usually yellow, sometimes cream. The inferior, syncarpous, bicarpellate ovary bears a single basal ovule per carpel (basal placentation). The fruit is a small achene bearing a minute pappus or a reduced scale crown, promoting wind‑assisted dispersal (Oberprieler et al., 2015).

The centre of diversity lies in the western Mediterranean, notably Spain and Morocco, where several narrow endemics occupy limestone outcrops. One species is confined to the Montseny range of Catalonia, another to the Rif Mountains of northern Morocco. The genus occurs from sea level to roughly 1,500 m, preferring well‑drained soils and full sun.

Flowers attract a variety of generalist insects, and achenes are mainly wind‑dispersed; where the pappus is reduced, occasional barochory may occur. Cytological work reports a base chromosome number of x = 9, with documented counts of 2n = 36 in several species (García et al., 2020).

Early treatments placed the genus within Tanacetum, but molecular phylogenetics consistently places Plagius as a distinct clade within Anthemideae, sister to the core Tanacetum complex (López‑González & Hidalgo, 2021). No formal subgeneric classification has been proposed, and recent taxonomic updates retain the genus at its current circumscription (POWO, 2024).

Several species are grown as ornamental rock‑garden plants for their silvery foliage and prolific capitula, while a few have become naturalized weeds in disturbed Mediterranean pastures.

Habitat loss from agriculture and overgrazing threatens several narrow endemics, and a forward‑looking assessment calls for targeted population surveys and ex‑situ conservation measures to secure the genus’s long‑term persistence.

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