Plexus was a French language magazine, started under the auspices of Planète science fiction magazine to which the late Pierre Bourgeade contributed.
Planète (The Planet) was a French fantastic realism magazine created by Jacques Bergier and Louis Pauwels. It ran from 1961 to 1972.
See also: plexus, http://journaux-anciens.chapitre.com/PLEXUS.html
Do these magazines have any value?
DMcC
About 20€ a piece.