200 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 3 



the requirements of the kelps, at least through most of the northern re- 

 gions, but the extremes of midsummer transform all surface waters into 

 tropical ones. There seems little question that conditions are adequate for 

 dispersal and introduction of these plants into the Gulf, but they have 

 not become established because they have not been able to survive the un- 

 favorable seasons. 



The richest development of algae in the Gulf is found on the reefs of 

 islands lying in the northern regions in the path of the rapidly moving, 

 more highly oxygenated water of those parts : San Esteban Island, Pond 

 Island, Puerto Refugio, Turner's Island, and Tepoca Bay, Sonora. Pos- 

 sible ecologic explanations have already been discussed. 



The shore vegetation of Turner's Island reef was as luxuriant in 

 January as was that of any other station. Collecting was again done in 

 July, under identical tidal conditions, exactly six months after the winter 

 visitation. The most significant observations have already been described 

 (Dawson, 1941, pp. 117-118). Here, it need only be repeated that with 

 the temperature increase of 15° C. there occurs a profound change in the 

 vegetation. Many winter elements apparently disappear completely or are 

 present only as juvenile forms. Some obscure elements of the winter flora 

 attain very rich and dominant development. Other entirely new elements 

 appear. The summer and winter floras are of more or less comparable bulk 

 but are of utterly different composition. Further mention of specific cases 

 may be found under Dictyota Johnstonii and by referring to the list of 

 species from Turner's sland on p. 357. 



The sublittoral floras of the Gulf as known from dredgings at a 

 limited number of stations appear to be exceedingly varied and apparently 

 often very rich. A large proportion of the sublittoral species were ob- 

 tained only by dredging, and the infrequent duplication of species in dif- 

 ferent hauls suggests how incomplete our knowledge of these plants is. At 

 Puerto Refugio, for instance, five dredge hauls were made in different 

 parts of the small bay. Each one yielded a number of species not found in 

 any of the others. Many such dredge hauls, in fact, have brought to light 

 unique samples of species unknown elsewhere. 



