Definition of guano. : a fertilizer containing the accumulated excrement of seabirds or bats; broadly : excrement especially of seabirds or bats.
This is a cross section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). Since columnar cells are quite thick, they do not readily allow passive diffusion.
The principle structural types are simple and stratified. Simple epithelium has only one layer of cells, while stratified epithelium has two or more layers. The form and function classifications of epithelial tissue are cuboidal, squamous, and columnar. Squamous cells have a plate-like shape and typically function in diffusion and protection.