NO. 10 DAWSON : MARINE ALGAE, GULF OF CALIFORNIA 193 



life in this region but to show wherever possible how the nature of the 

 vegetation reflects directly the peculiar conditions existing in this body 

 of water. 



The southwestern limit of the Gulf is taken as Cape San Lucas, the 

 outjutting rocks of the extreme tip of Lower California forming a geo- 

 graphic point of division between the two parallel coasts of the peninsula. 

 It is not possible to establish an eastern boundary, for no known collec- 

 tions of algae have been made on the mainland coast of Mexico between 

 Mazatlan and the mouth of the Rio Mayo. On the chart of collection 

 stations a 300-mile blank exists without a single record, and, instead of 

 attempting to extend the treatment southward across this unknown re- 

 gion, the collections at Rio Mayo are taken as representing the southern- 

 most samples known from the eastern Gulf coast. 



Geographically, the Gulf of California extends through ten degrees 

 of latitude and is, for this reason alone, subject to a very considerable 

 range of variation in climate. The southern waters lie below the Tropic 

 of Cancer and, as will be shown below, are influenced by temperature 

 conditions entirely unlike those acting upon the northern waters. Ac- 

 tually, the Gulf forms a marine pocket reaching deeply into the western 

 side of Mexico. Surrounded as it is by barriers to the tempering influences 

 of the Pacific winds, the waters of much of this marine area are subjected 

 to much greater climatic extremes than are the waters along the outer 

 coast of Lower California. On the west is the backbone of the peninsula, 

 a largely unbroken chain of mountains 6,000 to 10,000 feet high, barring 

 the access of winds which would cool the heated air of summer and stay 

 the falling temperatures of winter. On the north and east lies the great 

 Sonoran desert mass, flanked by the North American cordillera. The 

 summers of this whole country are notoriously hot and snow falls on the 

 cactus-studded desert in winter. The upper half or more of the Gulf, 

 therefore, confined by these remarkable barriers, is subjected to conditions 

 much as a large inland lake would be, and the temperatures of its surface 

 waters vary directly with these seasonal climatic conditions. 



Aside from the unusual temperature conditions existing in the region, 

 another remarkable feature is present which both directly and indirectly 

 has considerable influence on the distribution of the marine plants. No- 

 where else on the Pacific coast do the tidal conditions compare with those 

 at the head of the Gulf. The great length and relatively narrow propor- 

 tions of the Gulf result in the development of a tidal bore which reaches 

 tremendous volume in its northern regions. Twice a day the great influx 



