Ultrastructure and composition of cell wall appositions in the roots of Asplenium (Polypodiales)

► Cell wall appositions appear at the contact site with micro-organisms in fern roots. ► Infecting microbes are immobilized when attempting to penetrate intracellularly. ► Cell wall appositions are made of a non-lignified polysaccharide matrix. ► Callose and lignins are absent, yellow-brown phenolic compounds are present. ► They create a barrier protecting the root against infections.