DISTRIBUTION OF VEGETATION IN UNITED STATES. 59 



CUMULATIVE OCCURRENCE OF CHARACTERISTIC GRASSES. (PLATE H.) 



On plate 11 have been laid the distributional limits of 4 grasses 

 which are widespread and dominant in the Grassland region of the 

 United States. These species are Bouteloua oligostachya, Bulhilis 

 dadyloides, Bouteloua hirsuta, and Kceleria cristata. The most wide- 

 spread of these grasses is Koeleria, which is found throughout the 

 Northern States from Maine and Pennsylvania to Washington and 

 California, extending southwest into Texas. Bouteloua oligostachya 

 is also a plant of extensive range, having its limit in the Grassland- 

 Deciduous Forest Transition on the east and occurring locally as far 

 west as the northern Rockies and the desert plains of Utah, Arizona, 

 and southern California. The other species are more nearly coincident 

 in their distribution with the Grassland and Desert-Grassland Transi- 

 tion. The area of maximum occurrence in which all 4 species are 

 found extends from the Canadian boundary to the Rio Grande, 

 stretching approximately from the ninety-sixth to the one hundred 

 and fourth meridian. 



EXTREME LIMITS OF TWO TYPES OF CACTI. (PLATE 12.) 



The most widely distributed genus of cacti in North America is 

 Opuntia, in which a large diversity of types are to be found which have 

 been roughly grouped in the tw^o subgenera, Cylindropuntia and 

 Platyopuntia. In plate 12 we have shown the extreme range of these 

 two types of cacti in the United States. The limit of thp range of the 

 platyopuntias is the limit of the family Cactacese. This limit is 

 carried from the southwestern United States, in which plants of this 

 type are so abundant, eastward to the Atlantic coast and northward 

 in the Coastal Plain to the Northeastern States. Members of this 

 type are absent from the Allegheny region, but are found in eastern 

 Kentucky, southern Michigan, and southwestern Minnesota. This 

 limit is formed by the extreme ranges of Opuntia opuntia and Opuntia 

 polyacantha. In the Western States the limit of this type is found at 

 the base of the northern Rocky Mountains and at ths eastern foot of 

 the Cascade and Sierra Nevada ranges, being formed chiefly by the 

 extreme occurrence of Opuntia polyacantha and its closely allied forms. 



The arborescent cacti comprised in the group of Cylindropuntia 

 are much more closely restricted to the desert region of the South- 

 western States. The limit of this type is shown in plate 12 and is 

 formed on the east by the extreme range of Opuntia arhorescens, on the 

 north, in Nevada, by Opuntia acanthocarpa and 0. echinocarpa, and 

 on the west by Opuntia parryi (hernardina) , and 0. prolifera. 



