In biology, columnar refers to the shape of epithelial cells that are taller than they are wide. Form follows function in biology, and columnar morphorphology hints at the functions of the cell. Columnar cells are important in absorption and movement of mucus. The cells may or may not bear microvilli (involved in maximizing the surface area for intestinal absorption) or cilia (involved in moving mucus and trapped material up the respiratory passages to be expectorated or swallowed). Columnar epithelium may be simple or stratified. Simple columnar epithelium is most common and involves one layer of cells attached to a basement membrane. The nucleus is closer to the basal aspect of the cell than the apical aspect. Single stratification tends to indicate absorptive function. Stratified columnar epithelium is rare but can be found in salivary glands. It consists of a layer of columnar epithelium resting on top of at least one other layer of epithelial cells, which may have any shape (columnar, cuboidal, or squamous). Stratification in cuboidal tissue has a secretory function.