86 THE VEGETATION OF THE UNITED STATES. 



here concerned solely with the southern limits of distribution of these 

 plants, all of which range northward into the forested belt of Canada. 



19. Plants of Southern Transcontinental Range. (Plate 30.) 



Spermolepis echinata (Nutt.) Hell. 



Daucus pusillus Michx. 



Parietaria debilis Forst. ( = P. floridana Nutt.) 



Similar ranges extending from the Atlantic to the Pacific are found 

 in the case of a few herbaceous plants which grow and mature during 

 the different portions of the year in different parts of their transcon- 

 tinental ranges. Daucus extends from North Carolina through 

 Louisiana, Texas, and California, and up the Pacific coast to Washing- 

 ton, although it is relatively infrequent at the extremes of this range. 

 All of these plants extend beyond the limits of the United States and 

 we are able to investigate only the northern Umits of their distributions. 



20. Herbaceous Plants of Southwestern Range. (Plate 31.) 



Kallstroemia grandiflora Torr. ( = Tribulus grandiflorus Wats.). 



Cladothrix lanuginosa Nutt. 



Pectis papposa Harv. and Gray. 



Euphorbia serpyllifolia Pers. ( = Chamaesyce serpyllifolia (Pers.) Small. 



A small group of plants has here been selected as representing types 

 of distribution applying to a very large number of plants in the south- 

 western arid regions. Cladothrix, Kallstroemia, and Pectis are all con- 

 fined to the extremely warm regions of Texas, New Mexico, and 

 Arizona. Euphorbia is found throughout the western half of the 

 United States in varying abundance. 



21. Herbaceous Plants of Central Distribution (Nyctaginace^). 



(Plate 32.) 



Boerhaavia erecta L. 



Oxybaphus nyctagineus (Michx.) Sweet ( = Allionia nyctaginea Michx.). 

 Oxybaphus angustifolius (Nutt.) Sweet ( = Allionia hnearis Pursh). 

 Oxybaphus floribundus Chois. ( = AlHonia floribunda (Chois.) Kze.). 



In view of the fact that we have been concerned in so many cases 

 with only one of the two edges of distribution of plants, we have here 

 selected a group of central occurrence so as to make it possible to 

 investigate the conditions of their eastern and western limits. Mem- 

 bers of the same family have been chosen in this case because of the 

 desirability of working out the behavior of a group of plants which are 

 closely related in growth-form as well as in taxonomic relationship. 

 Boerhaavia is found in the extreme south from Georgia to Arizona, 

 while the other species of this group have their main regions of occur- 

 rence in the Grassland and Grassland-Deciduous Forest Transition. 



