Geographical Distribution of Sea.-Orassfin. 1 87 



VIII. TJtr North-Tnnpcrale group: 



Zostera marina and Zostera nana. 



Tliis <,M-()up has really (uily one typical representative — viz. : Z. 

 mariitti, but the other wide-spread species, Z. na?ia, comes perhaps 

 also best in here, as an irregular member. Its distribution, to- 

 gether with that of the Australian Zosferas, gives, I think, the key- 

 to the evolution of the genus. 



Zostera is a very much reduced type, which is supposed to have 

 come from Potamofjrton- and Rupjna-Wke ancestors. The flower- 

 ing axis bears flowers on one side only, and the flowers are naked; 

 but on the outer side of each flower we find a scale — the so-called 

 r e.f iliac iihi III. This scale is sometimes supposed to be a reduced peri- 

 anth. It is present in Zostera nana, and apparently also in the 

 Australian species, while wanting — ordinarily — in Z. marina. Its 

 presence in Z. nana, and allied species, seems to indicate that 

 they are tlie older types, and Z. marina the younger. The distri- 

 bution of Z. nana and of its allies strengthens this view. Z. nana 

 is known along the coasts of Europe, from Southern Scandinavia 

 southwards into tlie Mediterranean, where it penetrates into the 

 Black Sea; further, it is found on the Atlantic coast of Morocco 

 and in the Canaries. It does not occur along the tropical west- 

 coast of Africa, but re-appears in South Africa and in Madagas- 

 car. It is reported with doulit from the Seychelles also. Lastly, we 

 have records of it from Tonkin and Japan. Now it must be ad- 

 mitted that the identity of the plants from all these regions is not 

 certain, as it is known, with flowers and fruits, from Europe only, 

 and tlie vegetative organs show no specific distinctions. 



Owing to the uncei'tainty of some of the records, I find it better 

 to err on the side of caution, and to liiake the more general state- 

 ment : — that a narrow-leaved and small Zostera has been found 

 in the above-mentioned areas. I would associate with this form 

 the closely-allied three Australian species, the distribution of 

 which 1 have already given. In this way we get one group of 

 narrow-leaved and small Zosteras, wdth an almost word-wide dis- 

 tribution. It is worth noting that this gi-oup is absent from both 

 coasts of North America, and that the records are very scanty as 

 regards tropical localities. The distribution has some resemblance 

 to that of the genus Posidonia, but it is less restricted. And I 

 think the same explanation holds good here — viz. : the Zostera 

 group originated in the Tropics, and migrated both northwards 

 and southwards, nearly disappearing in its original home. Mean- 



