SECT. XXVI. THE GRASSY SEA. 253 



almost entirely from those of Suez and the Eed Sea. It is ob- 

 served that shallow seas have a different set of plants from such 

 as are deeper and colder ; and, unlike terrestrial vegetation, the 

 algse are more numerous in the mean latitudes than either 

 towards the equator or the poles. They vary also with the 

 depth: completely different kinds affect different depths, their 

 seeds being of such specific gravity as to remain and germinate 

 where the parent plant grew. The quantity of algse in that 

 accumulation known as the sargassa or grassy sea is so great, 

 that the early navigators, Columbus and Lerius, compared it 

 to extensively inundated meadows : it impeded their ships, and 

 alarmed their sailors. It is in the North Atlantic, a little to the 

 west of the meridian of Fayal, one of the Azores, between the 

 25th and 36th parallels of latitude. A smaller bank lies between 

 the 22nd and 26th degrees of north latitude, about 80 leagues 

 west of the meridian of the Bahama Islands. These masses 

 chiefly consist of one or two species of sargassa, the most ex- 

 tensive genus of the order Fucoidese. 



Some of the seaweeds grow to enormous lengths, and all are 

 highly coloured, though many of them must grow in deep water. 

 Light, however, may not be the only principle on which the 

 colour of vegetables depends, since Baron Humboldt met with 

 green plants growing in complete darkness in one of the mines 

 at Freyberg. 



In the dark and tranquil caves of the ocean, on the shores 

 alternately covered and deserted by the restless waves, on the 

 lofty mountain and extended plain, in the chilly regions of the 

 north, and in the genial warmth of the south, specific diversity is 

 a general law of the vegetable kingdom, which cannot be ac- 

 counted for by diversity of climate ; and yet the similarity, 

 though not identity, of species is such, under the same isothermal 

 lines, that if the number of species belonging to one of the great 

 families of plants be known in any part of the globe, the whole 

 number of the flowering or more perfect plants, and also the 

 number of species composing the other vegetable families, may 

 be estimated with considerable accuracy. 



Various opinions have been formed on the original or primi- 

 tive distribution of plants over the face of the globe ; but, since 

 botanical geography has become a science, the phenomena ob- 



