180 C. H. Ostenfeld: 



(2) 5 genera of Fofamogetonaceae — viz. : C ymodocea, Diplarv 

 thera, Posidonia, Zostera, and Phyllosjjodix. The tAvo first and 

 the two last respectively are closely related to each other. Thus- 

 it might be possible to classify these 5 genera also into three 

 groups, each group widely differing from the others. 



The systematic position of tlie sea-grasses, their aquatic habitat, 

 and the great structural differences between the genera, all point 

 to their great phylogenetic age. 



The natural agent of dispersal of the sea-grasses is the sea cur- 

 rents, as the fruits in no case ripen above the water. The Potamo- 

 getoriaceae have droupe-like fruits, the epicarp of which is a 

 rather thin, fleshy cover of inconspicuous colour. The Hydro- 

 charitaceae have a berry, also of inconspicuous colour, containing 

 either many small seeds (//alojdrila). oi- a few large ones {Enhalus^ 

 and y // ala.su io). In Zosfera (of the Potamogetonaceae) the epi- 

 carp, but in Enhalus and Thahisx/a tie -whole j-eritarp splits- 

 open, and ill all the genera the " stones," or seeds, then drop out. 

 As the fruits or seeds of the .sea-grasses do not possess a floating 

 apparatus, they are always heavier than water, and consequently 

 the migration of the species must be very slow. 



Besides tliis transportation by means of currents, it is possible 

 — but not very probable— that certain fishes, and sea-mammals, 

 such as the Dugong and Manatee, and turtles, may eat the fruits 

 and thereby contribute to their distribution. 



Tlie dispersal by means of detached shoots, which float in the 

 water, is almost negligible, as they do not live long when floating. 



Being green antotiophous plants, the sea-grasses can grow 

 only in shallow water where the light is sufficient for assimilation 

 — i.e., they grow only along the coasts. The depth at which they 

 grow varies in the different species and in different seas. In 

 northern seas (cf/., Davish), the deeper limits of .^'os^^/y/ lie at about 

 10 meters. In tlie Mediterranean, J'oxido/iia grows at greater- 

 depths, though I cannot accept without further evidence the record 

 of its occurrence at a depth of about ")() meters. 



The slow manner oi dispersal and the necessity of sliallow water 

 are factors which restrict the ai-ea of occun-ence of the sea-grasses. 



Of the 8 genera mentioned above, Enhalus is monotypic. It 

 occurs in the wliole of the tropical Indopacifir region. Tlip Zostrra 

 genus consists of two or more sjiecies. 



With the exception of PJn/llo.spadi.i, a younger type derived from 

 Zostera, and restricted to the northern Pacific, each genus inhabits. 



