252 MARINE PLANTS. SECT. XXVI. 



hitherto been observed in southern Africa, and, from what has 

 been already stated, the proportion of European species in Equi- 

 noctial America is still less. 



Islands partake of the vegetation of the nearest continents ; 

 but, when very remote from land, their floras are altogether 

 peculiar. The Aleutian Islands, extending between Asia and 

 America, partake of the vegetation of the northern parts of both 

 continents, and may have served as a chain of communication. 

 In Madeira and Teneriffe, the plants of Portugal, Spain, the 

 Azores, and of the northern coast of Africa, are found ; and the 

 Canaries contain a great number of plants belonging to the 

 African coast. But each of these islands possesses a flora that 

 exists nowhere else ; and St. Helena, standing alone in the midst 

 of the Atlantic Ocean, produces only two or three species of 

 plants recognised as belonging to any other part of the world. 



It appears from the investigations of M. de Humboldt that 

 between the tropics the plants, such as grasses and palms, which 

 have only one seed-lobe, are to the tribe which have two seed- 

 lobes, like most of the European species, in the proportion of one 

 to four ; in the temperate zones they are as one to six ; and in 

 the Arctic regions, where mosses and lichens, which form the 

 lowest order of the vegetable creation, abound, the proportion is 

 as one to two. Annuals with one and two seed-lobes, in the 

 temperate zones, amount to one-sixth of the whole, omitting the 

 cryptogamia (N. 218) ; in the torrid zone they scarcely form 

 one-twentieth, and in Lapland one- thirtieth part. In approaching 

 the equator the ligneous exceed the number of herbaceous plants ; 

 in America there are 120 different species of forest trees, whereas 

 in the same latitudes in Europe only 34 are to be found. 



Similar laws regulate the distribution of marine plants. Groups 

 of algae, or marine plants, affect particular temperatures or zones 

 of latitude and different depths, though some few genera prevail 

 throughout the ocean. The polar Atlantic basin to the 40th 

 degree of north latitude presents a well-defined vegetation. The 

 West India seas, including the Gulf of Mexico, the eastern coast 

 of South America, the Indian Ocean and its gulfs, the shores of 

 New Holland, and the neighbouring islands, have each their dis- 

 tinct species. The Mediterranean possesses a vegetation peculiar 

 to itself, extending to the Black Sea ; and the species of marine 

 plants on the coast of Syria and in the port of Alexandria differ 



