On the origin of lignin in the cell wall

1. A survey is given of the chemical composition of the various layers building up the cell wall in young, as well as in adult, stages. 2. An enumeration is given of the different and often discrepant theories on the origin of lignin. 3. In sunflower stalks at various growth stages determinations were carried out of lignin, cellulose, pectin, furfuralyielding components and of some substances not belonging to the wall. 4. The properties of the lignins isolated from these plant materials were studied and compared with the genuine lignins by means of microchemical reactions. Young lignins (from young parts of the stalk) are less polymerized than old lignins (from old parts of the stalks). 5. During the process of ageing and lignification the cellulose content remains nearly constant. The contents of lignin and pentosan increase while the pectin percentage is rapidly decreasing. These results are observed in a single stalk, proceeding from top to base, as well as in the same internode at successive growth stages. 6. Microscopic sections of the stalks were examined by application of microchemical reactions. 7. A close relationship is found to exist between pectin and lignin. If the pectin reactions are distinct, the lignin reactions are absent, and vice versa. 8. Special methods are described to determine the oxidationreduction potential of living tissues of higher plants (in our case of the sunflower). 9. Especially the cambium and the adjacent layers of young xylem elements have a very strong reduction power. 10. An attempt has been made tot draw up an acceptable theory on lignin formation. According to this idea pectin should be converted into lignin, this being a strong reduction process. The factors needed for the realization of this reduction are present. 11. A part of the pectin, not taking part in lignin formation, may be changed into hemicelluloses (pentosans).