[Proc. Rot. Soc. Victoria, 27 (N.S.), Part II., 1914.] 



Art. XIV.— O/j. the. Geographical DistrihvAion of the Sea- 

 Grasses. 



A Premminaky Communication. 



By 0. H. OSTENFELD 



(Copenliag-en, Denmark). 



Ooimuiinieated by Professor A. J. Ewar 

 [Read October 8tli, 1914.] 



For several years I have been studying the geographical distri- 

 bution of the Marine Flowering Plants, as it seemed to me that it 

 miglit have some general value hoth from a phylogenetic and a 

 geological point of view. 



We owe most of our kiiowledge on these plants to the late P. 

 Ascherson, who, during a series of years, contributed to their 

 study, and A\as much interested in their geographical distribution. 



Some considerations will show why I think this phytogeographical 

 study may be of more general interest. 



The marine flowering plants — or sea-grasses — belong to two 

 monocotyledonous families — viz.. Hydrocharitaceae and Potamo- 

 (jetonaceae, Iwth of which are included in the cohort Helohime. 

 Tliis cohort consists of several families, nearly all of which are 

 water-plants (or swamp-plants). They are very distinct from the 

 other monocotyledons, and undoubtedly represent old types. 



Sometimes it has been urged that from this cohort most of the 

 othei- Monocotyledons and Dicotyledons have originated. Be that 

 as it may, the Helobieae fni'm an especially well-marked group 

 within which the families show a progi'ession from types with 

 many fi'ee superior carpels to otheis with one syucarpous infei'ior 

 ovary. In all the families, genera witli one or a few species pre- 

 vail, and within the whole cohort only one genns. Potamogeton (the 

 Pondwoed). is really rich in species. It seems as if, in most cases, 

 the developing power of the genera has been checked. 



The sea-grasses belong to — 



(1.) 3 genera of Hydrocharitaceae — viz.: Halophila, Enhalus 

 And Thulassia, all widelv different from one another: and to 



