In general, outermost bark layer is considered to be a replacement for the epidermis of the stem and root, as the plant axis becomes older and grows thicker. What would occur if the cells on the outer zone of the cylinder, increasing in radius, did not keep pace via new cell divisions? The circumference of the axis would increase greatly, and the likely outcome would be stretching and tearing of the outer cell layers. In fact, this can be observed. A plant could experience extra cell divisions of the existing surface layers, to thereby increase the surface area, but this generally is not the case. Instead, seed plants, especially dicotyledons and gymnosperms, have solved this problem by producing new layers of cells (periderm) to accommodate tangential growth (lateral expansion) of the stem or root while, at the same time, helping to reduce water loss from the stem or root by forming the water-repellent cork cells.
The functions of the outer bark have been speculated about, but there has been relatively little scientific testing. Bark, and cork in particular, are excellent for thermal insulation, and thick bark is particularly common on trees in cold biomes, whereas thin barks are more typical of tropical habitats. During fires, thermal insulation by thick bark, or bark that does not burn into inner bark, can be an adaptation for plant species inhabiting fire-adapted communities.
Cork is indigestible, creating a surface that is not very attractive to many animals, although many herbivores (mammals and boring insects) attack the inner bark, where the secondary phloem is loaded with sucrose. Cork, with its fat and wax, is fairly water repellent, and thus creates a resistance to water loss from the plant to a drier atmosphere.
Occurrence of dead cells, without nutritional value, on the outer surface likely is a strategy to discourage growth of fungi and bacteria. However, in very wet habitats, such as cloud forests and rain forests, algae, bryophytes, lichens, and fungi may grow luxuriantly on wet bark, and branches can support huge loads of epiphytes.