198 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 3 



substratum is concerned, and where temperature conditions in February 

 were apparently satisfactory, an extreme paucity of algal vegetation ex- 

 isted. The same paucity was observed at Escondido Bay. Determinations 

 of oxygen values in the vicinity of these two localities show that the sur- 

 face oxygen content is low, first because of generally higher surface tem- 

 peratures, but principally because of definite upwellings of adjacent abys- 

 mal waters with a negligible oxygen supply. Moreover, wave action and 

 general water movement are exceptionally slight in both of these quiet, 

 well-protected bays. It seems, therefore, possible, and even probable, that 

 at these localities the development of marine vegetation is limited by oxy- 

 gen deficiency. 



General Aspects of the Algal Flora 



It has already been pointed out that the waters of the southern re- 

 gion of the Gulf of California are essentially tropical in nature. Like 

 most tropic waters, their littoral flora, as revealed by collections at San 

 Gabriel Bay, Espiritu Santo Island, are dominated by widely distributed 

 warm-water elements and are very scanty in bulk. Although San Gabriel 

 Bay is the only well-known station of its kind in the Gulf, its flora 

 may undoubtedly be considered typical of many such bays of the warm 

 waters of the southwestern Gulf coast and islands. The temperature of 

 surface waters varies less than 10° C. annually, and a uniform flora of 

 diminutive plants is prevalent. On the bottoms of the shallows grow 

 large coral heads, in and about which may be found Dictyosphaeria, 

 Gelidiella, and Caulerpa. Both Caulerpa racemosa and C. sertularioides 

 are abundant and often form thick mats. Several other warm-water 

 genera are present, Hydroclathrus, Wurdemannia, and Jania being fre- 

 quent. Sargassum was not encountered here, though it is known to occur 

 in the general region of La Paz. 



One who is not familiar with tropical marine floras but has seen the 

 rich algal growths of cool, northern latitudes would be immediately im- 

 pressed by the relative paucity of vegetation. Almost all tidal rocks are 

 completely barren, for very few plants of the southern Gulf seem to 

 tolerate exposure at low tides. On the whole, considering its poverty 

 and the dominance of tropical elements, the San Gabriel Bay flora shows 

 almost no characters in common with floras known for the northern part 

 of the Gulf. 



The shore flora of Guaymas harbor and near-by bays is moderately 

 well known for the winter season through the studies made on the Velero 

 cruise. The vegetation is not particularly rich or varied, but certain 



