Excerpts from Jim Conrad's
from the May 29, 2011 Newsletter issued from Mayan Beach Garden Inn 20 kms north of Mahahual, Quintana Roo, México
BIG TANGLES OF DODDER
Often along roadsides you see large, orange tangles spreading atop many kinds of plants, as shown above. There the tangle covers a dense population of Coastal Ragweed. A close-up showing the tangler's wiry, orange stems, some spikes of tiny, hardly open, white flowers and, in the upper, right corner, a white fruit, is below:
Folks into wildflowers up North will recognize this as a dodder, of which there are many species. Weakley's flora covering the US Southeast lists about 18 species for that region, but here in the Yucatán we appear to have only this very common one, sometimes known as American Dodder. It's CUSCUTA AMERICANA, spottily distributed throughout much of tropical and subtropical America, including southern Florida.
The thing about dodder is that it's parasitic. It's a member of the Morning Glory Family, so presumably its ancestors were similar to present-day morning glories. Once they evolved the practice of robbing water and nutrients from plants they grew on they no longer needed leaves and chlorophyll for photosynthesis, so they lost both, transforming themselves into nothing but orangish stems sometimes bearing flowers and fruits.
Dodder steals its food by wrapping around a plant host and producing rootlike appendages known as haustoria that penetrate the host's stem, tapping into the stem's vascular system. You can see haustoria emerging from the side of a twining dodder stem below:
Once the dodder plant has tapped into a host, the dodder's original roots in the soil die, leaving the dodder totally dependent on stealing sap. Up North dodder plants are annual but here in the tropics they can grow more or less continuously, often growing high into trees or covering swaths of vegetation much larger than in our picture. Some dodder species contain a little chlorophyll and can photosynthesize at low levels while other species have no chlorophyll at all and are entirely dependent on their parasitism. American Dodder's stems are sometimes green so apparently it does some photosynthesizing.
Studies have shown that dodder seedlings are more likely to grow toward plants releasing certain chemicals than plants not releasing those chemicals.