A tap root is a specific type of root structure present on some plants, which is typified by being rather large, and going directly down into the ground. This structure has other roots growing off of it as it descends, acting as a sort of trunk beneath the ground. Plants with tap roots can be contrasted with plants that have a fibrous root system.
There are a number of ways in which a tap root can be beneficial to a plant. One of the most important is that it allows the plant to reach down quite far to find water to sustain itself. In drier climates, or areas where water tends to run deep, this can be incredibly useful. Indeed, many desert plants have incredibly well-developed tap root systems, allowing them to survive in even the most arid of climates. Mesquites, for example, have adapted to survive in the Mojave Desert, and so have come up with many biological tools to help them reach and conserve water, including tap roots that can reach lengths of more than 80 feet (25m).
In many plants, this root structure may also function as a reservoir for food and water. It can grow very wide, and remain relatively protected underground, allowing the plant to save up energy for times when it may need it, such as when producing seeds. Many plants that use their tap root as a source of food are also eaten by humans, and the so-called root vegetables are generally plants with this type of root structure. Carrots, turnips, radishes, parsnips, jicama and burdock are all examples of commonly-eaten plants where the main part of the plant eaten is the tap root itself.
Generally these foods are not only quite nourishing, but are also extremely wet, because of the excess water the plant has been storing in them. It’s not difficult to imagine any of the examples above as being essentially water storage units. In fact, some plants, like jicama, seem to be almost like forms of solid water, the water content is so high.
Both a tap root system and a fibrous root system start as the same sort of root, and change a bit into the plant’s development. The first root that a plant sends out is called a radicle. In a tap root system, the radicle continues to push downwards and grow out, while sending out occasional small branches. In some cases these branches will scarcely be visible at all, as anyone who’s seen a carrot or turnip fresh from the ground knows. In a fibrous root system, on the other hand, the radicle will eventually fall away, to be replaced by a web of smaller roots.
Because of the size and depth of tap roots, plants that have them can be particularly difficult to get out of the ground. Transplanting plants with this type of root is notoriously more difficult than plants with a fibrous root system, and in many cases is almost impossible. Fully eliminating plants with a tap root can be difficult as well, because pulling up the entire root system may not be feasible. This is often seen with dandelions, which have very long and hardy tap roots. People quickly notice that when trying to pull dandelions up to remove them from a garden, they seem to grow back year after year. This is because a part of the root has remained in the ground, and a new plant eventually grows from it.