Saturday, June 11, 2011

Vegetarian Vampires

There is a particularly interesting killer on the farm. It creeps silently toward its prey, searching victims by their shadows and their smell. It kills its victims gradually, sometimes over months, slowly drinking their fluids until there are none left and the dessicated victims expire from exhaustion.

Terrified? Don’t be. These predators are vegetarians – vegans even. They are actually parasitic plants called dodder, in the genus Cuscuta. picture They are the ghostly bleached cousins of morning glories. Dodders comprise approximately 150 closely-related species of flowering vines with no green chlorophyll. Their appearance reminds me of a childhood poem I once loved about spaghetti trees, because they drape over their victims with vast quantities of whitish-yellow strands.

Dodder produces flowers, fruits and seeds like any normal plant. Their flowers are small white clusters that develop in to fruits with multiple seeds. The seeds drop in groups onto the soil to germinate up to 5 years later. Even as the seeds germinate, the plant seems normal. The seedlings grow using the energy stored inside the seed by the mother plant. Since seedlings of most plant species grow from stored energy, it is common for seedlings to be white or yellow like those of dodder. Most plants quickly grow leaves that fill up with chlorophyll, and they start making their own food before they run out of what’s stored in the seed. But here is where dodder is unique.

If dodder seedlings don’t find a victim within 10 days of germination, they will starve. They are somewhat picky eaters, as dodder won’t parasitize grasses or corn. Fortunately, they have evolved the ability to find their preferred foods. The seeds germinate faster when a preferred host is near. They grow toward host stems by growing toward shade (most plants grow toward light), and by curling when they touch something. The most amazing feature of dodder is that it can find its hosts by smell. Dodder can detect and grow toward the volatile compounds produced by its food. Ever smell a tomato plant? Dodder has too!

When dodder finds a host, it punctures the stem and grows into the food veins, or phloem, of the plant. It absorbs water, sugar, minerals and everything else it needs straight from the plant. It discards its own stem and root connecting it to the ground and goes completely airborne as it quickly covers its host plant with spaghetti stems.

Dodders are annuals, meaning they die back each year and regrow from seeds. Some, however, have figured out another way to survive the winter. If they colonize a host plant that survives the winter, the dodder growing inside the host stem can survive and resprout the next spring.

The best parasites don’t usually kill their hosts, because then they would be out of a food source. This is usually true with dodder too. It may kill some of the plants it is in contact with, but it usually maintains at least one live host. It certainly reduces host plant size, which is a problem for farmers when the host plant is a crop plant. Farmers don’t like dodder, and they often have to work hard to get rid of it. On the farm where I work, we hand-pull the dodder and throw it onto mowed grass, where it can’t regrow. Desperate farms can switch to non-host plants for a few years in particularly infested fields.

As much as I know I should hate the plant, I can’t quite. It’s just so darn strange. There are very few truly parasitic plants, and this is one. People always think Venus Fly Traps are the consumers of the plant world, since their little traps look so much like mouths, and they move (!), but sadly, they are not consumers. They are green, indicating chlorophyll, and they get all their energy from the sunlight. They do, however, absorb nitrogen, which is like a plant vitamin, from the flies they trap. Dodder, though, has discarded its ability to photosynthesize. The loss of a trait is fascinating from an evolutionary perspective. Organisms evolve new traits all the time, but they don’t usually lose them. Of course, dodder probably still has most of the genes necessary for photosynthesis, because it is so closely related to and descended from photosynthesizing plants. But there is really no need to spend the energy to maintain those genes. The dodder plants that spent more energy parasitizing host plants and less energy maintaining their ability to photosynthesize found the successful strategy.

Are their any plants that are parasitic on animals? No. But the laws of natural selection say that will be is variation. Doubtless there is some dodder plant out there that might just maybe be able to puncture human flesh and collect its energy from me. So you’ll find me moving just a little bit faster past the dodder plants!


  1. And to think I have been telling 5th graders for years that a plant is defined as a living mulit-celled organism that gets the vast majority of its energy by producing it from light energy, various gasses, water, and some nutrients. I always enjoyed dispelling the myth that a Venus Flytrap must then not be a plant. I had no idea that there were fully parasitic plants.

    So, what is a plant?

  2. Sorry to be so late in replying, but better late than never. Here are the characteristics that all plants share. Plants all have cell walls made of cellulose. They are autotrophs that photosynthesize, or they are the evolutionary descendants of plants that photosynthesize. They are eukaryotic organisms. That's about it!