MARIXE ALGAL FLORA 261 



American and on the European coasts by no means necessarily correspond. 

 A good example is Dictyota dichotoma, which in America reaches 35° 

 N.L. but in Europe extends at least 23° farther north, into decidedly 

 colder water. It is the American strains of these species which concern 

 us, and their American distribution, not the similar, but not necessarily 

 physiologically identical, European representatives. 



It is customary to think of our western tropical Atlantic algal flora 

 as typified by what appears in the Caribbean. However, the Caribbean 

 Sea lies far north of the Equator. Its highly complex shoreline, with a 

 host of islands and reefs, favors a rich algal flora, which focusses atten- 

 tion on it. The very brief northward extension of this flora to 30° X.L. 

 in Florida and 32° N.L. in the Bermudas is not unexpected, for the Gulf 

 Stream, flowing through the Straits of Florida, favors just such an ex- 

 tension. The almost total disappearance of the flora in the Gulf of 

 Mexico as one goes north and northwest is one of impoverishment, and 

 not of replacement by a temperate flora. Muddy shorelines and large 

 injections of fresh water, rather than the somewhat lowered temperatures 

 of winter, seem to be the chief reasons for this. The t>'pe of algal vege- 

 tation does not show any distinctive change as we go southeast across the 

 Equator. Allowing for our less adequate exploration of the coast, it is 

 seen to retain the same character far toward the southern boundary' of 

 Brazil. Extensive regions are probably so affected by the discharges from 

 the Orinoco, the Amazon, and other great rivers that they are nearly 

 sterile, and other regions with muddy shores are similarly unproductive. 

 Where we have collections in the Guianas and northeastern Brazil, Ave 

 find no change of character, no incursion of a new and more equatorial 

 tropical element, in spite of the northwestwardly tending South Equa- 

 torial Current. Furthermore, perhaps aided by the southerly Brazil Cur- 

 rent, the same type of flora extends many hundreds of kilometers farther 

 to the south, and only when we reach southern Brazil do new elements 

 begin to appear. Not until we reach Uruguay at about 35° S.L. do the 

 new elements dominate, though they still do not altogether replace species 

 typical of the Caribbean. 



A general chart (Table 1) shows many of the features of the Carib- 

 bean flora. The number of species in each class which we may accept as 

 well established appears in the first column of numbers, and the second 

 column shows the species that have been reported enough times to give 

 us a clue to their probable range. In the third column this is broken 

 down for each algal class into four categories, depending on whether the 

 alga is limited to the Caribbean in American waters, ranges northward, 

 ranges southward, or is wide-spread in both directions from the Carib- 



