INVOLVEMENT OF EXODERMAL PASSAGE CELLS IN MYCORRHIZAL INFECTION OF SOME ORCHIDS

Roots with velamen of three orchids were examined for mycorrhizal infe ction: an epiphytic orchid Stanhopea tigrina, a lithophyte Dendrobium kingianum and a terrestrial Epidendrum radicans. Approximately 50% of all exodermal cells were passage cells that remained non-lignified. Lo ng cells of the exodermis were lignified, but not suberized. A similar proportion of passage cells was infected in young and old root sectio ns. Most coils were located in the three cell layers of cortex beneath the exodermis. A larger proportion of coils was collapsed in the olde r part of the roots of D. kingianum and E. radicans.