Cuscuta

Annuals or perennials, parasitic, usually lacking chlorophyll; roots rudimentary and ephemeral. Stems trailing or twining, attached to host by haustoria, greenish, white, yellow, orange, or pink-purple, filiform, glabrous. Leaves sessile; blade reduced, scalelike, 1–2 mm, surfaces glabrous. Inflorescences cymes or thyrses; pedicellate or not; bracts 0–1(–11). Flowers (3 or)4 or 5-merous, thickened-fleshy to thin-membranous; sepals ± connate, calyx usually campanulate, cupulate, or cylindric, rarely angled or slightly zygomorphic, 2–6 mm; corolla usually white, white-cream, or yellowish, sometimes pink or purple, cupulate to cylindric, 2–6(–9) mm , limb (3 or)4- or 5-lobed; infrastaminal scales usually present, dentate or fimbriate; styles 1 or 2; stigmas 2, ± globose or ± elongate. Fruits capsular, depressed-globose to ovoid, indehiscent or dehiscence circumscissile. Seeds 1–4, glabrous, seed coat honeycombed when dry and papillate when wet or seed coat epidermis cells rectangular and not honeycombed/papillate; hilum terminal, subterminal, or lateral; embryo uniformly slender, 1–3-coiled, rarely globose-enlarged at base; cotyledons absent. x = 4.

Distribution

nearly worldwide, most diverse in the Western Hemisphere, especially warmer regions.

Discussion

Species ca. 200 (51 in the flora).

Cuscuta is classified within Convolvulaceae; its outgroup relationships are not clear (S. Stefanović et al. 2002; Stefanović and R. G. Olmstead 2004). Pollen and gynoecium features suggest that Cuscuta is allied with Dicranostyloideae clade or Convolvuloideae Burnett (M. Welsh et al. 2010; M. A. R. Wright et al. 2011).

Some Cuscuta species parasitize numerous hosts; others have relatively narrow host ranges. Some of the former species can be weeds especially destructive to crops, including alfalfa, carrots, chickpeas, clover, cranberries, fava beans, flax, lespedeza, potatoes, tomatoes, and sugar beets. When growing on perennial hosts, haustoria of some Cuscuta species overwinter inside their hosts and generate new growth in spring (J. H. Dawson et al. 1994; M. Costea and F. J. Tardif 2006; K. Meulebrouck et al. 2009).

Seed dispersal has been considered unspecialized (J. Kuijt 1969); endozoochory by migratory birds was recently documented in some species (M. Costea et al. 2016). Worldwide dispersal of weedy Cuscuta species has been through contaminated seeds of forage legumes, especially alfalfa, clover, and lespedeza.

Identification of most Cuscuta species is usually a lengthy process: rehydration, dissection, and examination of flowers with a microscope may be necessary. For stem diameters, the following categories are used (T. G. Yuncker 1921): slender = 0.35–0.4 mm‚ medium = 0.4–0.6 mm, and coarse = greater than 0.6 mm. Measurements of floral parts were done on rehydrated mature flowers. Lengths of flowers were measured from base of calyx to tip of a straightened corolla lobe. Color of calyx and texture of calyx and corolla were noted only on dried flowers. Observation of papillae and multicellular appendages require magnifications of at least 50×. Orientation of corolla lobes (erect, spreading, reflexed) was noted in mature flowers. Dehiscence or indehiscence of capsules can be predicted early in development of ovaries; lines of dehiscence are readily detectable at the bases of young ovaries; the ovary wall will tear along the dehiscence line when light pressure is applied. Capsule walls sometimes rupture irregularly.

Three of the four subgenera recognized in Cuscuta are known from the flora area; members of subg. Pachystigma (Engelmann) Baker & C. H. Wright are endemic to South Africa (M. Costea et al. 2015).

Selected References

None.

Lower Taxa

Cuscuta subg. Cuscuta, Cuscuta subg. Grammica, Cuscuta subg. Monogynella

Key

1	Inflorescences thyrsoid: paniculiform, racemiform, or spiciform; styles usually 1, sometimes distally 2-fid or separating into 2; seed coat epidermis cells rectangular, not honeycombed when dry and papillate when wet.	Cuscuta subg. Monogynella
1	Inflorescences monochasial cymes: corymbiform, fasciculate, glomerulate, paniculiform, racemiform, ropelike, spiciform, or umbelliform; styles 2; seed coat epidermis cells honey­combed when dry and papillate when wet.	> 2
2	Styles equal; stigmas clavate, cylindric, or terete.	Cuscuta subg. Cuscuta
2	Styles unequal; stigmas globose.	Cuscuta subg. Grammica