Strand and Dune Flora of the Pacific Coast 175 



break in Mertensia across arctic Siberia, which may never have 

 been occupied; in Lathyrus there is a gap across arctic Canada 

 as welh 



It is o£ interest to note that upon the coast of southern Alaska 

 and British Columbia, where glaciation must have completely 

 exterminated plant life, all four species have effected a fairly 

 thorough reoccupation. They grow wherever suitable habitats 

 occur. This has been possible because they are primarily inhab- 

 itants of the upper beach and not dependent upon accumulations 

 of dune sand. 



All four species have given rise to varietal forms in different 

 parts of their extensive ranges, the most important of which have 

 been noted in a previous section. Doubdess some of these have 

 arisen in connection with the Pleistocene interruption of their 

 ranges; satisfactory treatment of this problem must await more 

 detailed knowledge. 



2. Species with close relatives on the Atlantic coast of North 

 America and in central Alaska and eastern Siberia. 



The two species of Tanacetum comprise this small group. 

 They resemble the species of the first group, especially Elymus 

 and Lathyrus, in being represented by very closely allied forms 

 on the Atlantic coast of North America, the Great Lakes, and in 

 Alaska and eastern Siberia. The group, however, does not extend 

 to Europe and is not present on the arctic and subarctic coasts 

 of America. Moreover, the species of Alaska and Siberia is not 

 coastal. The point of origin cannot be determined, but it seems 

 clear that a formerly continuous range was broken up by gla- 

 ciation. The present-day forms may well have arisen through 

 isolation. 



3. Species occurring also on the coasts of eastern Asia. 



This group comprises two species with remarkably similar dis- 



