The leafless seedlings of dodder die soon after emerging from the ground unless they happen to touch and then attach to a suitable host plant. These rapidly growing parasites may spread from one species to another above ground, twining their white or yellowish to orange stems over their hosts, to which they become attached by numerous small haustoria (“suckers”). Once a dodder is united by its haustoria, the first portion of the stem withers and the plant is no longer connected with the ground; thereafter, the parasite obtains all its food and water from its host. Our species are all annuals, and mostly native. Only a few species of the genus (including some introduced in North America as seed contaminants) cause serious damage to crop plants, but the whole genus is tarred with the appellation of “noxious weeds.”
Measurements of flower length are taken from the base of the perianth to the tips of the corolla lobes. Numbers of perianth parts are not always constant, and several flowers should be examined to learn the usual number for a given specimen. Counts are often more easily made on fresh material.
1. Stigmas slender, linear, not enlarged; styles (including stigmas) much longer than ovary and fruit; fruit circumscissile near its base; plants introduced, on legumes.
1. Stigmas capitate; styles mostly no longer than the ovary; fruit breaking open irregularly, not circumscissile; plants native, on a wide variety of hosts.
2. Sepals separate, subtended by bracts of similar shape and texture (but with outcurved tips); flowers sessile, in very dense, usually rope-like masses wound around the host.
2. Sepals united basally, not subtended by bracts; flowers sessile or pediceled, but the inflorescence not rope-like.
3. Flowers all or mostly with calyx and corolla 4-lobed; fruit usually becoming ± depressed-spherical at maturity.
4. Lobes of corolla obtuse or rounded at tip, much shorter than the tube; calyx mostly shorter than corolla tube; mature styles ca. (0.7–) 1–1.5 mm long, slightly shorter than the fruit; old dry corolla often persistent on summit of fruit.
4. Lobes of corolla acute, triangular (or appearing rounded because of incurving in C. coryli), about as long as or longer than the tube; calyx about equaling or surpassing corolla tube at flowering time; styles and old corollas various.
5. Mature styles ca. 1 mm or slightly longer; calyx lobes acute; old dry corolla persistent on summit of fruit, or falling off; opening at summit of fruit ringed with a thickened ridge; lobes of calyx and corolla papillate [20×].
5. Mature styles less than 1 mm long; calyx lobes obtuse to rounded; old dry corolla persistent around base of fruit; opening at summit of fruit not ringed with a thickened ridge; lobes of calyx and corolla smooth (not papillate).
3. Flowers all or mostly with calyx and corolla 5-lobed; fruit various (usually spherical to slightly elongate in C. gronovii and C. indecora and depressed-spherical in C. pentagona and C. campestris).
6. Lobes of corolla broadly rounded, spreading to reflexed at maturity, the tips not incurved (or occasionally so in drying).
6. Lobes of corolla triangular, acute, their tips usually ± incurved (though lobes themselves may spread).
7. Corolla lobes papillate [20×]; calyx usually shorter than corolla tube; inflorescence rather open, the pedicels generally longer than the flowers; perianth ca. 2.5–4 mm long; old withered corolla persistent around fruit.
7. Corolla lobes not papillate; calyx as long as corolla tube; inflorescence more compact, the pedicels generally shorter than the flowers; perianth ca. 1–2.5 mm long; old withered corolla persistent at base of fruit.
8. Calyx smooth, not angled or knobbed; perianth ca. 2–2.5 mm long.
8. Calyx of most flowers appearing angled or knobbed below each sinus; perianth ca. 1.5–2 mm long.