A plant tissue whose cell walls have become impregnated with lignin. Due to the added strength that this confers, sclerenchyma plays an important role in support; it is found in the stems and also in the midribs of leaves. The cell walls contain pits, enabling the exchange of substances between adjacent cells. Mature sclerenchyma cells are dead, since the lignin makes the cell wall impermeable to water and gases, and so they occur in parts of the plant that are no longer growing lengthways. Sclerenchyma cells take the form of fibres or sclereids. Compare collenchyma; parenchyma.