Each cork cambium produces a sector of cells called a periderm. The dividing cells (initials) of the cork cambium (phellogen) divide to produce cells, most of which are "pushed" toward the outside, and some may be pushed toward the inside. Cells pushed to the outside generally have layering of suberin in their walls, and thereby become cork cells (also called phellem). Suberin, often composed of suberic acid and phellonic acid, is a fatty and waxy substance, which makes the cells mostly impervious to water and unable to exchange gases and nutrients, hence these cells soon die and entrap air. In some species, cells pushed to the outside of the cork cambium can develop with thickened cell walls but lacking suberin (phelloids). Cells pushed from the cork cambium to the inside are called phelloderm, and these can remain alive because they have unthickened and unspecialized cell walls and, hence, can exchange gases and obtain nutrients.