The spore of Equisetum germinate shortly after shedding and gives rise to form prothallus. Prothalli of Equisetum are basically monoecious and the gametophytes are rarely seen. The simplest known sporophyte among bryophytes is that of Riccia while highest evolved known is Bryopsida (Polytrichum, Pogonatum). Sexual reproduction in Volvox is oogamous type. The spores in slime molds (e.g., Physarum) are haploid, unicellular, usually globose with smooth, spiny or reticulately thickened cell walls.