“…The presence of lignin imparts rigidity to the plant, allowing the xylem to maintain a high hardness to support the plant's weight. [10][11][12] Second, at the molecular level, lignin is complex and heterogeneous, consisting of three structural units: p-hydroxyphenyl, guaiacyl, and syringyl subunits. The relative contents of these monomeric units are related to many factors, such as plant species, growth cycle, extraction method, etc.…”