The root system of most plants is underground and serves as anchorage and for absorption of water and nutrients. The primary root originates in the embryo and is the first structure to emerge from the seed. Primary roots are usually positively geotropic i.e. they respond to gravity by growing downward.

      Like the shoot, the root has an apical meristem. This root apical meristem (RAM) is covered with a protective root cap. This cap protects the root tip and the RAM as they grow down through the soil. The RAM produces cells toward the root body that differentiate into the vascular, ground, and dermal tissues of the root system. The RAM also adds new cells to the root cap.

      While the vascular tissue in the shoot system is positioned around the circumference of the shoot, the vascular tissue is located in the central stele of the root. New lateral roots arise in the pericycle that surrounds the central column of xylem and phloem. Many first order lateral roots do not respond to the forces of gravity and grow parallel with the ground surface as a means to finding an abundant source of water. Root hairs are formed by epidermal cells that extend outward from the root. This provides a huge amount of surface area to take up water. The root's region of elongation has no root hairs protruding from the epidermis because they would easily be torn or sloughed off as the root pushes down into the soil.

Allorhizic vs. Homorhizic Radication

      Allorhizic refers to the fact that there are spatially separated and distinct shoot and root systems. The embryos of gymnosperms and dicotyledonous angiosperms are bipolar with a root or radicle pole directly opposite the shoot pole. The pteridophytes are homorhizic and exhibit no primary root pole and therefore lack the development of a separate root system. Their roots are initiated by the shoot system.

      Monocotyledonous angiosperms are homorhizic as a result of the precocious death of the primary root that forms in the embryo. All subsequent roots that form on monocotyledons originate from the stem. These shoot-borne roots are called adventitious roots. Shoot borne roots may originate solely at nodal positions as seen in Zea mays, or they may originate internodally, as seen in Hedera helix, a dicot.

Tap Root and Fibrous Root Systems

      In gymnosperms and dicots, the primary root becomes a taproot that produces lateral roots as it grows downward into the soil. In this type of root system, the older lateral roots are located near the soil surface.

      Since the primary root in monocots is short-lived, some develop a system of adventitious roots with many lateral branches. This fibrous root system does not grow as deeply into the soil as a taproot system but the fibrous nature of the system enables them to stay well rooted in shallow soil and prevent erosion.