The reddish-brown
substance lignin is the actual filling and lignifying substance of the plant which,
due to its swelling effect (less than with cellulose) and cementing and stiffening
effect is responsible for the compressive stability of the plant. Thus, a high lignin
content ensures that the sprout of a plant will remain upright and can dry off even
with a decreasing water pressure as is the case, for example, with the stalks of ripe
cereal plants in the field. Wooden biomass types have much higher lignin contents
than one year old plants (annual harvesting cycle) and can therefore also withstand
high continuous stresses in spite of relatively low water contents. The lignin
content also varies within a plant, depending on the type of biomass. Thus, the
highest lignin values are typical of the lowest, highest and innermost parts of a
trunk, of softwood branches, bark and compression wood1.1 Being an organic
substance, the natural lignin depletion begins with the harvesting of the plant/
felling of the tree and can be accelerated by externnal factors (UV light, heat and
moisture).