Cuscuta pentagona is not your normal plant. it is a spindly orange vine that can grow up to three feet high, produces tiny white flowers of five petals and is found all over North America. What is unique about Cuscuta [commonly known as dodder] is that it has no leaves. And it isn’t green, because it lacks chlorophyll, the pigment that absorbs solar energy, allowing plants to turn light into sugars and oxygen through photosynthesis. Cuscuta gets its food from its neighbors. It is a parasitic plant. In order to live, Cuscuta attaches itself to a host plant and sucks off the nutrients provided by the host by burrowing an appendage into the plant’s vascular system. What makes Cuscuta truly fascinating is that it has culinary preferences: it chooses which neighbors to attack.
A Cuscuta seed germinates like any other plant seed. The new shoot grows into the air, and the new root burrows into the dirt. But a young dodder left on its own will die if it doesn’t quickly find a host to live off of. As a dodder seedling grows, it moves its shoot tip in small circles, probing the surroundings the way we do with our hands when we are blindfolded or searching for the kitchen light in the middle of the night. While these movements seem random at first, if the dodder is next to another plant (say, a tomato), it’s quickly obvious that it is bending and growing and rotating in the direction of the tomato plant that will provide it with food. The dodder bends and grows and rotates until finally it finds a tomato leaf. But rather than touch the leaf, the dodder sinks down and keeps moving until it finds the stem of the tomato plant. In a final act of victory, it twirls itself around the stem, sends microprojections into the tomato’s phloem (the vessels that carry the plant’s sugary sap), and starts siphoning off sugars so that it can keep growing and eventually flower.
Consuelo De Moraes even documented this behavior on film. She is an entomologist at Pennsylvania State University whose main interest is understanding volatile chemical signaling between insects and plants and between plants themselves. One of her projects centered on figuring out how Cuscuta locates its prey. She demonstrated that the dodder vines never grow toward empty pots or pots with fake plants in them but faithfully grow toward tomato plants no matter where she put them—in the light, in the shade, wherever. De Moraes hypothesized that the dodder actually smelled the tomato. To check her hypothesis, she and her students put the dodder in a pot in a closed box and put the tomato in a second closed box. The two boxes were connected by a tube that entered the dodder’s box on one side, thereby allowing the free flow of air between the boxes. The isolated dodder always grew toward the tube, suggesting that the tomato plant was giving off an odor that wafted through the tube into the dodder’s box and that the dodder liked it.