Cortex

Cortex, a tissue-system, differentiated at an early period in the development of the axis of most flowering plants, between the dermatogen or rudimentary epidermis and the plerome or fascicular and medullary portion. In its embryonic stage it is termed the periblem. As it passes partly into permanent tissue, its outer, hypodermal or sub-epidermal, layers of primary cortex frequently become collenchymatous, i.e. acquire a mucilaginous thickening of the corners of their cells. The inner cells may remain rounded and loosely-aggregated with intercellular spaces, and often contain stores of starch, whilst the whole tissue retains protoplasm and undergoes the cell-divisions necessary to accommodate it to the increasing girth of the axis. A layer of its cells remaining merismatic is known as the phellogen, and is in exogenous stems the usual source of the periderm, secondary cortex, or cork (q.v.). The cells beneath the phellogen generally retain their copious store of chlorophyll, which made the shoots green, even after they are buried beneath many layers of opaque brown cork. These green cells are the phelloderm. The elements of cortex generally remain parenchymatous, but isolated cells may become woody (sclerenchymatous), or be filled with crystals, and resin-passages and laticiferous vessels also occur in cortex.