LIgnin is a substance found in vascular plants, usually within the cell walls and also between cells themselves. It is largely a supportive structure and is part of the secondary thickening of tall plants. It is a major constituent of ‘woody’ material. Most trees would topple over without lignin supporting their tissues, in addition to the long, dead cells of the xylem vessels. When secondary thickening is formed it is called lignification.
Surprisingly, although a dead tissue, lignin is incredibly strong and can help support tall trees of several hundered tonnes whilst maintaining its integrity. Lignin also is not simply a passive material. It often has tiny pores called lenticels through it through which the plant cells beneath in the epidermis can take in gas for gaseous exchange in respriation and also it may have specialised links called tracheids which link xylem vessels to other cells.
It plays a part, along with the xylem vessels, along whose walls it is usually deposited, in controlling the transportation of liquid in plants. It prevent the walls of the xylem vessels collapsingunder pressure and adds strength to the woody material of an older plant.
Lignin is not simple instructure and in fact, different plants have different kinds of lignin. It is usually composed of amino compounds and bonded in complex ways to carbohydrate molecules in the wood. Older plants with woody material which we eat like cabbage become ‘stringy’ and taste woody becuse the layers of lignin have got too thick over time.
Lignins are of different types including those which are in different plant genera of soft, hard and grasses. the structure of lignin varies from plant species to plant species and even from one tissue to the next in plants. Their precise function is not yet clear but they effectively strengthen bonds to increase stability of water transport vessels and also keep them open to allow the passage of water, dissolved nutrients and waste products from the plant.
Though useful to plants as they increase in size and girth, lignin does have its drawbacks. Beans, for example become enedible as they age due in part to lignin deposits and as paper is made, used and recycled, lignin tends to remain as it is hard and difficult to dissolve so each time you recycle paper, the content of lignin increases and this impairs the quality of the paper. Finding bio-friendly ways of removing it without resorting to chemicals like dioxin is a challenge for the future.
