Spores of the fern Onoclea sensibilis L. normally germinate to produce two cells of unequal size. The larger cell divides to produce the familiar heart-shaped prothallus. The smaller cell elongates and differentiates into the rhizoid but normally does not divide again. Onoclea spores germinate in complete darkness. Dark germination can be completely inhibited by ethylene gas (10 microliters per...

Disjunction between East Asian and eastern North American plants has been recognized since the time of Darwin. Although there is considerable evidence for congruent vicariance associated with late Neogene/Quaternary cooling among angiosperms, similar studies specific fern families (e.g., Osmundaceae Martinov) have appeared incongruous this pattern. Pteridologists continue to debate whether long...