Noting that Ca2+, and the lanthanide praseodymium, inhibited IAA-induced elongation, whereas EDTA reversed the inhibitory activity of Ca2+, and even promoted growth, Bennet-Clark (1956) suggested that IAA acts as a Ca2+/Mg2+ chelator. This model proposed that IAA removes Ca2+ and leads to a loss of Ca2+ pectates, which are replaced by pectate free acids or methyl esters. The latter, because they are not cross-linked, would render the wall plastic and able to elongate (Bennet-Clark, 1956).