It is the the growth or movement of plants toward or away from water. and in roots they show display hydrotropism in growing towards a water source. 
Hydrotropism  is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration. A common example is a plant root growing in humid air bending toward a higher relative humidity level.This is of biological significance as it helps to increase efficiency of the plant in its ecosystem.The process of hydrotropism is started by the root cap sensing water and sending a signal to the elongating part of the root. Hydrotropism is difficult to observe in underground roots, since the roots are not readily observable, and root gravitropism is usually more influential than root hydrotropism. Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.