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Lignin is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. It provides strength to the plant parts. It is largely a supportive structure and is part of the secondary thickening of tall plants. It is a major constituent of woody material. The cells of sclerenchyma are dead, long and narrow because their walls are thickened due to lignin. It acts as a cement and hardens them. Often these walls are so thick that there is no internal space inside the cell.