G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed).
S, Optical section of part of thick-walled stereid of Phanerogam, with almost obliterated cavity and narrow slit-like oblique pits.
In many forms its hyphae are particularly thick-walled, and may strikingly resemble the epidermis of a vascular plant.
In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex.
In many cases the cells bordering the leaf are produced into teeth, and very frequently they are thick-walled so as to form a supporting rim.