Strand and Dune Flora of the Pacific Coast i8i 



peratures doubtless set potential limits to northward migration 

 of sensitive southern species, but south of the glacial boundary, 

 as I have pointed out, the effect of this factor is merely to con- 

 vert potential ever-growers into winter-dormant perennials. In- 

 creasing dryness southward may be a factor in the eUmination 

 of certain species. Those ranging farthest south are superficially 

 the most xeric. 



COMPARISON WITH THE ATLANTIC COAST OF NORTH AMERICA 



Two facts stand out clearly : likeness between the floras of the 

 two coasts in the north, and unlikeness in the south. All four 

 species in the subarctic group are present also on the northern 

 Adantic coast, together with a closely allied species of Tana- 

 cetum. Three of these five range farther southward on the 

 Adantic coast: Mertensia, io°; Honckenya, 7°; Lathyrus, 1°. 

 Elymus goes 5° farther south on the Pacific coast. The ranges of 

 Tanacetum on the two coasts are closely similar. There are but 

 two important strand plants on the northern Adantic coast that 

 are not native on the northern Pacific, and both of these have 

 become naturalized there. Cakjle edentula already behaves like 

 a native, and Atnmophila arenaria is fast making itself thor- 

 oughly at home. Four species of the northern Pacific coast are 

 not native upon the Adantic: Carex macrocephala, Juncus les- 

 curii, Fragaria chiloensis, and Glehnia littoralis. It is of interest 

 that the Asiatic form of Carex macrocephala has become estab- 

 lished in one or two places on the New Jersey coast and is spread- 

 ing rapidly (Fernald, 1930). 



The general strand flora of the southern Adantic coast differs 

 absolutely from diat of the southern Pacific. Five gtvitr^— Carex, 

 Polygonum, Atriplex, Croton, Oenothera— occur in common, 

 but the parallel is no more than a coincidence. 



