Convolvulaceae Juss.

Parasitic Genera Included: Cuscuta (Tourn.) L.

Habit: Mostly annual (some overwinter inside host tissue and re-emerge), twining, parasitic herbs, usually with little or no chlorophyll (see below).

Parasitism: Both hemiparasites and holoparasites occur in the genus. Molecular studies of the chloroplast genome and physiological studies of photosynthetic enzymes show that Cuscuta reflexa retains a minimal yet functional plastid genome (Haberhausen and Zetsche, 1994; Bömmer et al., 1993, Haberhausen et al, 1992). Conversely, the plastid genome of C. europaea has sustained greater losses (Freyer et al., 1995) and shows no RUBISCO activity (Machado and Zetsche, 1990). Cuscuta pentagona has higher growth rates on and may prefer hosts that have vesicular-arbuscular mycorrhizal fungal associations on their roots (Sanders et al. 1993, Can. J. Bot. 71:1143).

Roots: A radical forms upon seed germination but soon degenerates after aerial portions of the plant have established haustorial connections to the host. Mature plants have no connection to the ground. Upon contact with the host (or inanimate objects), dodder stems twine

Haustoria: Prehaustorial "bumps" form on the inner surface of the twining stems. Later, haustorial initials differentiate and push their way through the prehaustorium into the host tissue, eventually forming connections to the host stele (xylem and phloem).

Stem: Filiform (thread-like), commonly yellow or orange in color. Epidermis with stomata. Cortex of three layers with articulated laticifers (Lyshede, 1985, Nord. J. Bot. 5:65). Primary vascular tissue centrifugal. Vascular bundles usually five, small, colateral. Xylem of vessels with simple end-walls. Stem without intraxylary phloem. Hairs, if present, unicellular or bicellular, not glandular.

Leaves: Much reduced, scale-like, alternate, spiral, with anomocytic stomates.

Inflorescence: Cymose, flowers densely packed in short spikes or heads. Often forming near points of attachment to the host.

Plant Sex: Plants hermaphrodite.

Flowers: With or without subtending bracts. Perfect, actinomorphic, 5-merous (less often 3- or 4-merous).

Calyx: Free or connate at base; persistent on fruit.

Corolla: Sympetalous with imbricate lobes; with a whorl of fringed (fimbriated) or cleft scales in the tube beneath and alternating with the stamens. Corolla white or pink in color.

Androecium: 5 stamens adnate to corolla tube by their filaments; alternate with petal lobes. Scales alternating with stamens may be interpreted as staminodes. Anthers short, tetrasporangiate, dithecal, opening by longitudinal slits. Pollen 3-nucleate, papillate, 3-6 aperturate.

Gynoecium: Of two (rarely three) syncarpous carpels, hypogynous. Locules as many as carpels. Style terminal, distinct or united, some deeply cleft. Stigma dry, papillate, capitate or linear-elongated.

Ovule: Two per locule, basal-axile (or intruded parietal), anatropous, with a massive single integument, tenuinucellar. Polygonum-type embryo-sac development. Endosperm formation nuclear.

Embryo, etc. Chlorophyllous embryo filiform-cylindric, with an enlargement at one end, more or less without cotyledons; sometimes spirally wrapped around the starchy endosperm. Rudimentary at time seeds are dehisced.

Fruit: a circumsciissile near base or irregularly dehiscent capsule or fleshy and indehiscent.

Seed: With oily endosperm. Seed coat very hard, requiring mechanical or chemical scarification to induce germination. Seed size and shape often similar to host plants such as alfalfa (lucerne, Medicago sativa), thus favoring seed spread.

Chromosomes: X = 7 (sometimes 15).

Link to Family Description in Delta (Cuscutaceae)

SIUC / College of Science / Parasitic Plant Connection / Cuscutaceae
Last updated: 17-June-10 / dln