All eucalypts add a layer of bark every year and the outermost layer dies. In about half of the species, the dead bark is shed exposing a new layer of fresh, living bark. The dead bark may be shed in large slabs, in ribbons or in small flakes. These species are known as "smooth barks" and include "E. sheathiana", "E. diversicolor", "E. cosmophylla" and "E. cladocalyx". The remaining species retain the dead bark which dries out and accumulates. In some of these species, the fibres in the bark are loosely intertwined (in stringybarks such as "E. macrorhyncha" or peppermints such as "E. radiata") or more tightly adherent (as in the "boxes" such as "E. leptophleba"). In some species (the "ironbarks" such as "E. crebra" and "E. jensenii") the rough bark is infused with gum resin.