[1] Land plants maintain adequate tissue hydration by means of an outer waterproof layer.
In older, woody tissues, waterproofing chemicals are present in the secondary cell wall that limit or inhibit the flow of water.
[2] Animals maintain adequate tissue hydration by means of (1) an outer skin, shell, or cuticle; (2) a fluid-filled coelom cavity; and (3) a circulatory system.
[3] In humans, a significant drop in tissue hydration can lead to the medical condition of dehydration.
Administration of hydrational fluids as part of sound dehydration management is necessary to avoid severe complications, and in some cases, death.