Root hairs are unicellular and tube-like projections from the epiblema of the roots. These are specialised hair-like structures meant for the absorption of water. Absorption of water inside the soil by the root hairs can occur in two ways:
1. In the case of active absorption, water is absorbed by the roots with the expenditure of cellular energy, like ATP. Several hypotheses are postulated to support this method of absorption.
2. In the majority of the time, water is absorbed by the roots by creating a difference in the Diffusion Pressure (also called DPD). In the mesophyll of leaves, water loss causes a decrease in Turgor pressure (TP). To balance this fall in TP, DPD increases. With a higher value of DPD, mesophyll picks up water from surrounding tissue and creates a chain of pull or deficit. This pull reaches the xylem and eventually to the root's xylem. This deficiency is continued up to the epiblema of the roots. When the DPD of root hairs exceeds DPD of water in the soil, water passively enters root hair cells.
TP and OP are used to calculate the DPD of a cell.