20 INTEODUCTION. IV. 



the waters which flow over submarine sands will carry the spores of the Alga3 with 

 less injury than the winds of the desert will convey the seeds of plants from one 

 oasis to another. It cannot, however, be doubted that submerged sands do exercise 

 a very material influence on the dispersion of Algas, or their 



GEOGRAPHICAL DISTRIBUTION. 



Climate has an eflect on the Algse as upon all other organic bodies, though its 

 influence is less perceptible in them than in terresti'ial plants, because the tempera- 

 ture of the sea is much less variable than that of the air. Still, as the temperature 

 of the ocean varies with the latitude, we find in the marine vegetation a corre- 

 sponding change, certain groups, as the Laminarice, being confined to the colder 

 regions of the sea ; and others, as the Sargassa, only vegetating where the mean 

 temperature is considerable. 



These differences of temperature and corresponding changes of marine vegetation, 

 which are mainly dependent on actual distance from the equatorial regions, are 

 considerably varied by the action of the great currents which traverse the ocean, 

 carrying the waters of the polar zone toward the equator, and again conveying 

 those of the torrid zone into the higher latitudes. Thus, under the influence of the 

 warm Avaters of the Gulf Stream, Sargassum is found along the east coast of 

 America as far as Long Island Sound (Lat. 44°). And again, the cold south-polar 

 current which strikes on the western shores of South America, and runs along the 

 coasts of Chili and Peru, has a marked influence on the marine vegetation of that 

 coast, where Lessonia, Macrocystis, Durvillcea, and Iridcea, characteristic forms of 

 the marine flora of Antarctic lands, approach the equator more nearly than in any 

 other part of the world. 



The influence of currents of warmer water is also observable in the submarine 

 flora of the west coast of Ireland, where we find many Algte abounding in lat. 53°, 

 which elsewhere in the British Islands are found only in the extreme south points 

 of Devon and Cornwall. These, and other instances which might be given, are 

 sufficient to show that average temperature has a marked influence in determining 

 the marine vegetation of any particular coast. 



Seasons of greater cold or heat than ordinary have, as might be inferred, a cor- 

 responding action. This is particularly noticeable among the smaller and more 

 delicate kinds which grow within tide marks, and are found in greater luxuriance 

 or in more abundant fruit in a warm than in a cold season. And the difference 

 becomes more strongly marked when the particular species is growing near the 

 northern limit of its vegetation. Thus in warm summers, Padina Pavonia attains, 

 on the south coast of England, a size as large as it does in sub-tropical latitudes ; 

 while in a cold season it is dwarf and stunted. 



In speaking of the difference in colour of Alg£e, I have already noticed the pre- 

 valence of particular colours at different depths of water. A corresponding change 

 of specific form takes place from high to low water mark ; and as the depth 

 increases, the change is strikingly analogous to what occurs among land plants at 

 different elevations above the sea. Depth in the one case has a correspondent 



