Rhizodermis is the root epidermis (also referred to as Epiblem), the outermost primary cell layer of the root.
Specialized rhisodermal cells, trichoblasts, form long tubular structures (from 5 to 17 micrometers in diameter and from 80 micrometers to 1.5 millimeters in length) almost perpendicular to the main cell axis - root hairs that absorb water and nutrients. Root hairs of the rhizodermis are always in close contact with soil particles and because of their high surface to volume ratio form an absorbing surface which is much larger than the transpiring surfaces of the plant.
With some species of the Fabaceae family, the rhizodermis participates in the recognition and the uptake of nitrogen-fixing Rhizobia bacteria - the first stage of nodulation leading to formation of root nodules. Rhizodermis plays an important role in nutrient uptake by the plant roots. Due to an osmotic rupture of rhizodermal cells caused by osmotic shock, phosphate uptake from low concentrated solutions strongly decreases. Phosphate uptake from higher concentrated solutions is influenced slightly. Rhizodermal cells differentiate in the root meristem after asymmetric division from initial cells (Dolan et al.,1993). During rhizodermis development, the cells specialize further into two distinct cell types, rhizodermal cells without root hairs (atrichoblasts) and root hair-bearing cells (trichoblasts). Once initiated, root hair growth is characterized by oriented tip growth, compared to that in a growing pollen tube. The zone of growth is restricted to the tip of the growing cell (Sievers and Schnepf, 1981). As a consequence of oriented cell elongation, root hair extends into the yet-unexplored rhizosphere, where it acquires water and nutrients from the soil solution to sustain plant growth.