342. Hemizonia de Candolle in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 5: 692. 1836.
[Greek hemi-, half, and zona, belt or girdle, alluding to cypselae half enfolded by phyllaries]
Bruce G. Baldwin, John L. Strother
Annuals, 5–80 cm. Stems ± erect. Leaves mostly cauline; proximal opposite (forming winter-spring rosettes, sometimes present at flowering), mostly alternate; sessile; blades narrowly elliptic to linear or lance-linear, margins serrulate or entire, faces puberulent, hispidulous, hirsute, strigose, sericeous, or villous (distal leaves sometimes stipitate-glandular as well). Heads radiate, borne singly or in ± paniculiform, racemiform, or spiciform arrays or in glomerules. Peduncular bracts: pit-glands, tack-glands, and/or spines 0. Involucres hemispheric to ± urceolate or globose, 3–8+ mm diam. (usually subtended by calyculi of 5–7 bractlets in H. congesta subsp. calyculata). Phyllaries 5–14 in 1 series (linear to lanceolate or oblanceolate, herbaceous, each usually 1/2 enveloping a subtended ray floret proximally, abaxially pubescent to hirsute or villous, and stipitate-glandular). Receptacles flat to conic, glabrous, paleate (paleae connate, forming cells around all or most individual disc florets, scarious, ± deliquescent). Ray florets 5–14, pistillate, fertile; corollas white or yellow (often purple-veined abaxially). Disc florets 5–60+, functionally staminate; corollas white or yellow, tubes shorter than funnelform throats, lobes 5, deltate (anthers ± dark purple; styles glabrous proximal to branches). Ray cypselae ± obcompressed, abaxially gibbous, basal attachments oblique, apices sometimes beaked, beaks inconspicuous, straight, diameters greater than lengths, faces glabrous); pappi 0. Disc cypselae 0; pappi 0. x = 14.
Species 1: w North America.
Hemizonia as often circumscribed (to include Centromadia and Deinandra) is polyphyletic; molecular phylogenetic studies have indicated that Centromadia and Deinandra are more closely related to Calycadenia, Holocarpha, and Osmadenia than to Hemizonia in the strict sense, which is most closely related to Blepharizonia, also with 2n = 28 (S. Carlquist et al. 2003). Crosses between taxa of Hemizonia have yielded hybrids of minimal to full fertility; natural hybrids also have been documented (J. Clausen 1951). All taxa in Hemizonia are self-incompatible. The name H. congesta subsp. congesta has been sometimes misapplied to plants treated here as H. congesta subsp. lutescens; the type of H. congesta corresponds to plants that have been called H. leucocephala, a synonym of H. congesta subsp. congesta (B. G. Baldwin et al. 2001).
Babcock, E. B. and H. M. Hall. 1924. Hemizonia congesta: A genetic, ecologic, and taxonomic study of the hay-field tarweeds. Univ. Calif. Publ. Bot. 13: 15–100.