ABSTRACT
Despite the fact that cell walls were the first cell structures observed by simple microscopes, the apoplast was considered “the dead excretion product of the living protoplast” and thus received much less attention until the mid 1980s [1, 2]. Today, the apoplast is defined as all compartments beyond the plasma membrane, i.e., the interfibrillar and intermicellar space of the cell walls, the xylem as well as the gas-and water-filled intercellular space [2]. The outside boundary of the apoplast is formed by the rhizoplane and the phylloplane.
