Roots absorb water from the surrounding soil using the process of osmosis. Plant and tree roots have special structures, called root hair cells, that cover a large surface area and act as tiny sponges to absorb maximal amounts of water.
The number of root hair cells varies depending on the type of root system, as well as the size and species of the plant or tree. Generally, these cells coat the outer surfaces of branches, creating a layer of silk-like material. These cells help speed up the process of osmosis, which functions much like the process of photosynthesis. Instead of absorbing sunlight, however, roots absorb water, which is transported through the plant or tree for different purposes.
Some water is used to perform photosynthesis, while a portion is reserved for the purpose of enabling growth of leaves, stems and buds. Some water is used to help cool leaves; the warmer and drier the surrounding air, the more water that is used for this purpose. Roots also use osmosis to draw in key minerals and nutrients required for plant growth; the intake of water and supplements often occurs simultaneously.
As with water, plants may store or immediately use minerals upon absorption. Root hair cells are generally most active after rainfall, when water is plentiful. Plants can then assume dormant states during prolonged dry spells.