The Occurrence of a Hypersensitive Reaction in the Planktonic Diatom Asterionella Formosa Hassall Parasitized by the Chytrid Rhizophydium Planktonicum Canter Emend., in Culture*

S ummary Culture work has shown that cells of the diatom Asterionella formosa Hassall may undergo a hypersensitive death reaction in response to infection by the chytrid Rhizophydium planktonicum Canter emend. In the clone of Asterionella studied most of the infected cells exhibited this fatal reaction. On these incompatible cells the fungus died and the encysted zoospore did not enlarge into a sporangium. However, in very rare instances a compatible cell bearing a sporangium was found. An investigation of this mainly incompatible, hypersensitive host‐parasite relationship was conducted using light microscopy and experimentation.