Sclerenchyma is plant support-tissue, the cells of which have become rigid due to the formation of lignin. It occurs in the veins, stems, and midribs of leaves. As sclerenchyma cells mature, they die because the lignin in the thickened cell wall does not allow the passage of gases and water, although narrow channels (plasmodesmata) in the sclerenchyma act as intercellular cytoplasmic bridges to facilitate communication and transport of materials between cells.