Dodder is an unusual and unwanted plant that attacks other plants. Except for its flowers, the plant looks like spaghetti noodles. Its almost leafless, thread–like stems hang down atop other plants that dodder needs to stay alive. Dodder does not produce its own food. Instead, it steals it from other plants. It feeds by sucking juices from the plant it is wrapped around, often making its host very weak or even killing it. Dodder can find other plants by their smell. When a dodder seedling starts growing, it follows the scent of plants it prefers, like tomato plants, potato plants, or other farm crops.

   Unlike most plants that usually grow in the direction of light or warmth, a dodder plant will grow in the direction of, for example, tomato odor––if a tomato happens to be growing nearby.

   However, a young dodder plant must find a host plant quickly. If it cannot catch a whiff of a potential host within a few days, it will dry up and disappear—even if there is plenty of water around. Once it finds a host, the young dodder plant will attach itself to it and start growing faster. At that point, the dodder plant will drop its root.

   Dodder is thus a difficult weed to manage and a real headache for farmers. When it does get out of hand, dodder can greatly reduce a farmer’s harvest or even destroy crops completely. Before sowing their produce, farmers in warm parts of the world often check to make sure no unwanted dodder seeds have intermingled with their crop seeds. This is a good way to stop dodder plants from sneaking their way into a crop field.

What causes dodder to grow in a particular direction?

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