Gelation of pectin by plant extracts is due to demethylation by pectin methylesterase; in the presence of Ca—ions Ca-pectate gels are formed. Pectin methylesterase is inhibited to some extent by 1,2-dihydroxy-anthraquinone-sulphonic acid-3, 1,4-dihydroxy-anthraquinone-sulphonic acid-2, and 8-hydroxy-quinoline sulphate, but much stronger by m-digallic acid and tannic acid. A hmy-eliminative breakdown of pectin can interfere with the action of pectin methylesterase. In host-parasite complexes, the degradation of pectin (either hydrolytically or tranj-eliminatively) can be catalyzed by many different enzymes of plant, fungal or bacterial origin. Various substances, occurring naturally in higher plants, can greatly affect the activity of these enzymes. Therefore, in order to obtain quantitative data on their respective activities in host-parasite systems, thorough purification of all pectolytic enzymes from such systems is indispensable.