Cork Cambium and Secondary Growth

Increase in stem diameter through secondary growth is driven mainly by the vascular cambium, but that growth puts the original outer epidermis under strain, and it eventually ruptures — so it has to be replaced by a new protective layer. Cells of the outer cortex regain meristematic activity and organise themselves into phellogen, or cork cambium, which arises in the outer, extrastelar region of the stem as a layer of thin-walled, rectangular, actively dividing cells. Like the vascular cambium, phellogen cuts off new cells on both of its faces: cells produced toward the outside mature into phellem (cork), while cells produced toward the inside mature into phelloderm (secondary cortex). Cork is impervious to water because its cell walls are impregnated with suberin, and the secondary cortex it protects is itself parenchymatous in nature. Together, phellogen, phellem and phelloderm make up the periderm. As the periderm forms, the pressure it generates crushes and kills the tissues lying outside it; bark is the informal term covering everything external to the vascular cambium, including these crushed layers, the periderm and the secondary phloem — bark formed early in a growing season is comparatively soft, while bark formed later in the season is harder.

Scattered over the surface of the bark are small raised pores called lenticels — regions of the periderm where phellogen activity has been especially vigorous, leaving loosely packed cells that allow gas and water-vapour exchange to continue across an otherwise impermeable, suberised surface. …