Figure 3.
Fossil spores showing the diversity of the products of meiosis in early land floras. (a–f) Cryptospores from the Upper Ordovician (ca 450 Ma) of Oman. (a) Monad. (b) Dyad. (c) Tetrad. (d) Monad enclosed in an envelope. (e) Dyad enclosed in an envelope. (f) Tetrad enclosed in an envelope. (g) Unornamented trilete from the Lower Devonian (ca 415 Ma) of the Anglo-Welsh Basin. (h–i) Ornamented trilete from the Lower Devonian Rhynie chert (ca 407 Ma). Spores ca 20–30 µm diameter. First published in [4] (courtesy Charles Wellman). (Online version in colour.)