Below it is the conducting surface (B) of glassy epidermal cells, with short downward-directed points, which facilitate the descent, but impede the ascent of an insect.
The uppermost is a purely muscular cell from the sub-umbrella; the two lower are epidermo-muscular cells from the base of a tentacle; the upstanding nucleated portion forms part of the epidermal mosaic on the free surface of the body.
U.e., Upper epidermal cells, with (c) cuticle.
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.
The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage.
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