title: Parenchymatous cell division characterizes the fungal cortex of some common foliose lichens.
abstract: OBJECTIVE
  Lichen-forming fungi produce diverse vegetative tissues, some closely resembling those of plants. Yet it has been repeatedly affirmed that none is a true parenchyma, in which cellular compartments are subdivided from all adjacent neighbors by cross walls adjoining older cross walls.
  
  
  METHODS
  Using transmission electron microscopy (TEM), we tested this assumption by examining patterns of septum formation in the parenchyma-like cortex of three lichens of different phylogenetic affinities:Sticta canariensis,Leptogium cyanescens, andEndocarpon pusillum.
  
  
  RESULTS
  In the cortex of all three lichens, new septa adjoined perpendicularly or obliquely to previous septa. Septal walls possessed an electron-transparent core (median) layer covered on both sides by layers of intermediate electron density. At septal junctures, the core layer of the newer septum was not continuous with that of the older septum. Amorphous, electron-dense material often became deposited in the core region of older septal walls, and the septum gradually delaminated along its median into what could then be recognized as the distinct walls of neighboring cells. However, cells maintained continuity at pores, where adjacent remnants of the electron-transparent core layer suggested septal partition rather than secondary establishment of a lateral wall connection via anastomosis.
  
  
  CONCLUSIONS
  Although fungal tissues first arise by the coalescence of filaments early in lichen ontogeny, the mature cortical tissues of some lichens are comparable to true parenchyma in the unrestricted orientation of their septal cross walls and the resulting ontogenetic relationship among neighboring cell compartments.
authors.affiliation: Department of Biological Sciences, Florida Gulf Coast University, Ft. Myers, Florida 33965 USA wsanders@fgcu.edu.
authors.forename: William B
authors.initials: WB
authors.lastname: Sanders
authors.affiliation: Departamento de Bioquímica y Ecología Microbiana, Museo Nacional de Ciencias Naturales (CSIC), C/Serrano 115-dpdo, E-28006, Madrid, Spain.
authors.forename: Asunción
authors.initials: A
authors.lastname: de Los Ríos
mesh_terms: D001203:Ascomycota / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D002473:Cell Wall / Q000502:physiology / Q000648:ultrastructure; D008011:Lichens / Q000166:cytology / Q000382:microbiology* / Q000648:ultrastructure; D046529:Microscopy, Electron, Transmission; D013045:Species Specificity; D013559:Symbiosis / Q000502:physiology*
keywords: Endocarpon; Leptogium; Sticta; fungal cell wall; fungal tissue; lichen cortex; paraplectenchyma; pseudomeristem; pseudoparenchyma; septum; true parenchyma