I, End of hydroid of the thalloid Liverwort Blyttia, showing the thick lignified wall penetrated by simple pits.
The walls of these cells are strongly lignified (i.e.
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 whole of the middle lamella or originally formed cell-wall separating one from another disappears before the adult state is reached, so that the walls of the hydroids consist of a framework of lignified bars, with open communication between the cell cavities.
The former often has its cells lignified, and may consist of alternate layers of hard and soft cells.
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