Cliffs don't form in wet climates because they will become mud and eventually fall, causing a mudslide.
Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis.
This is really basic, containing much on the history of cell biology. The cell theory, or cell doctrine, states that all organisms are composed of similar units of organization, called cells. The concept was formally articulated in 1839 by Schleiden & Schwann and has remained as the foundation of modern biology.