DISTRIBUTION OF VEGETATION IN UNITED STATES. 79 



The members of this group are all shrubs of swamp and palustrine 

 habitat and have been selected for use in our work with a view to 

 bringing out the features of climatic control for plants which exist 

 under a practically uniform set of soil-moisture conditions. Adelia 

 is confined to the central portion of the Atlantic Coastal Plain. Itea 

 occupies a similar area, but extends throughout the Coastal Plain as 

 far north as New Jersey. Decodon is found in palustrine situations 

 throughout the eastern United States, and Cephalanthus occupies a 

 similar area with a remarkable westward prolongation of its range 

 into southern Arizona and the central valley of California. 



11. MiCROPHYLLOus Desert Shrubs. (Plate 21.) 



Artemisia tridentata Nutt. 



Covillea tridentata (DC.) Vail (including C. glutinosa (Engelm.) Rydb.). 



These shrubs have been used in view of the fact that they are the 

 most abundant plants in two of the most important of the desert 

 areas. Artemisia is found throughout the Great Basin Microphyll 

 Desert and extends beyond its limits to the east and northeast into the 

 higher portions of the Grassland area. Covillea is the leading plant 

 throughout the California Microphyll Desert and the Arizona and 

 Texas Succulent Deserts, extending eastward into the Texas Semi- 

 desert. 



12. Cacti. (Plate 23.) 



Opuntia polyacantha Haw. (including closely related varieties) . 



Carnegiea gigantea (Engelm.) Britton and Rose ( = Cereus giganteus Engelm.) 



The species of cacti which have been used were taken as examples 

 of this great group of stem- succulent plants, the former representing 

 one of the most widely distributed of the species found in the United 

 States, and the latter one of the most restricted of the species which 

 is in any place an important element of the vegetation. Opuntia poly- 

 acantha is found throughout the Grassland region, extending from 

 southern New Mexico to the Canadian boundary and occurring in 

 small areas in Washington. Carnegiea is limited to southwestern 

 Arizona at elevations below 4,000 feet. 



13. Characteristic Composites of the Great Plains. (Plate 24.) 



Silphium laciniatum L. 

 Solidago missouriensis Nutt. 

 Gutierrezia sarothrae (Pursh) B. and R. 



These suffrutescent perennials are found throughout the Grassland 

 and Grassland-Deciduous areas and, in their range and relations to 

 climate, may be taken as typical of a large number of similar plants. 

 Silphium occupies the most easterly range of this group of plants, 



