phellogen

This term is a highly specialized botanical descriptor used almost exclusively in plant anatomy and histology. It describes the cork cambium, a lateral meristem that allows woody plants to increase the girth of their stems and roots by producing protective outer layers.

Because it refers to a specific type of biological tissue, it is treated as an uncountable noun in most scientific contexts, referring to the substance or tissue type rather than individual units. It is rarely used outside of academic textbooks or professional botanical research.