Cells of the root cap are slowly replaced as they wear off, but the majority of root growth happens a few millimeters back from the tip. Secondly, additional root growth occurs farther up the root in an area known as the zone of maturation. In this area the root may send out lateral branches, in addition to very fine extensions called root hairs. The root hairs are extremely important to the plant for absorption of water and especially nutrients. The greater the surface area of the root the more it has contact with the surrounding soil and soil water.
Root hairs increase the surface area many times over.
If the all the surface of the roots, along with the root hairs, were added up it would represent more surface area than is found in all the above ground parts for many plants. Almost all plant species have fungi associated with their roots. Some of the fungi are parasitic and harmful to the plant, but a large number of special fungi, called mycorrhizae, are actually beneficial and may even be required for a plant to be healthy. The relationship between plant roots and mycorrhizae is referred to as symbiosis, which means that the plant and fungus mutually benefit one another. Many types of mycorrhizae are associated with a single plant species, whereas others may form relationships with a broader range of species.