An important structural component of the cell walls of plants, which gives the plant stability (in Latin, lignum means wood). A plant's lignin content increases as it matures. Lignin molecules are complex organic polymers, whose composition and structure varies between different plant species. Although lignin is a component of structural fibre in the horse's diet, it is not a carbohydrate, and it can only be broken down by specific enzymes produced by certain fungi and bacteria. It is therefore of very little nutritional value to the horse, and in fact most herbivores.