THE SMITHSONIAN INSTSTUTION. 107 



tliau 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 Alga?, 

 or their 



nKOGRAPTTTflAT. nTSTPTRUTTON, 



Climate has an effect on the Alg^M as upon all other organic bodies, 

 though its influence is less perceptible in them than in terrestrial 

 plants, because the temperature 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 corresponding change, 

 certain groups, as the Laminaricc, 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 ma- 

 rine vegetation, which are mainly dependent on actual distance from 

 the equatorial regions, are considerably varied by the action of the 

 greafe currents which traverse the ocean, carrying the waters of the 

 polar zone towards the equator, and again conveying those of the torrid 

 zone into the higher latitudes. Thus, under the influence of the warm 

 waters 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, Avhere Lessonia, 31a- 

 crocystis, 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 aver- 

 age 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 in- 

 ferred, a corresponding 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-trop- 

 ical latitudes ; while in a cold season it is dwarf and stunted. 



In speaking of the difference in color of Algfe, I have already noticed 

 the prevalence of particular colors at different depths of water. A cor- 

 responding 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. 

 D'epth in the one case has a correspondent effect to height in the other ; 



