The aquatic fauna is diverse, including the nationally scarce soft hornwort and fen pondweed. The plant body of a hornwort is a haploid gametophyte stage. The thallus cells often contain more than one chloroplast, as opposed to other hornwort genera. It seems to be associated with floating vegetation such as Hornwort and Water Milfoil ("Myriophyllum"). Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. Spiny hornwort can be found in ponds and lakes, principally in eastern North America. Aquatic plants include rigid hornwort, which is rare in London. The life of a hornwort starts from a haploid spore. Leiosporoceros dussii is the only species in the hornwort genus Leiosporoceros. Sphaerosporoceros is a genus in the hornwort family Anthocerotaceae. In either case, the protonema is a transitory stage in the life of a hornwort. There are a few localised examples of the Rigid Hornwort ("Ceratophyllum demersum"). Phymatoceros is the only genus in the hornwort family Phymatocerotaceae and order Phymatocerotales. This hornwort has a circum-boreal distribution across temperate Europe and North America. Hornwort plants float in great numbers just under the surface.