Roots are adapted for various environmental conditions. Roots anchor plants and are capable of absorbing water and minerals. Sometimes, in dry areas, such as grasslands, roots are in large mats just under the surface. These areas can be dry for long periods of time. Sometimes, even if the grass above the surface dies, the roots below the surface will propagate once again, when water becomes available. Cacti in the desert have very shallow root systems so that they can absorb water quickly before it evaporates, in the event of rain and store it in the fleshy stems. Trees in areas that have abundant water contain deep root systems. These will help to support the weight of the tree above ground and can go deep enough to tap into the water table. Aerating roots grow above ground in swampy areas where water is abundant. These are adapted for gas exchange. Roots can have root hairs, which are single cell projections off the main roots. These increase surface area to enable the roots to absorb a greater amount of water via osmosis.