PLANT LIFE IN OKLAHOMA. 43 



more {Platanus occidentales) , hickory {Oorya Illinoensis) , hickory {Carya 

 cordAformis,) hackberry ('Oeltis mississipfdensis) , blackberry {Rubus 

 villosus), iblack-cap raspberry (Rubus occidentalis) , dwarf chestnut oak 

 {Quercus prinoides), black oak (Que/reus veluntina), southern red oak 

 {Quercus texana), blue ash (Fraxinus quadrangulata) , persimmon 

 (Diofipryros virginiana) , winged elm {TJlmus alata), flowery dog wood 

 (Cormis florida). Several of these extend somewhat father westward 

 than the Pennsylvanian-Permian line in the southern part of the State. 

 Since the lines of equal precipitation swing westward toward the southern 

 end of the State, and the species named are found in their farthest west 

 stations in places suffering less from evaporation than the average condi- 

 tions of the regions (mountains, canyons, and river valleys), it seems 

 that the governing factor of their western limits is the amount of moisture 

 accessible in the soil and the relative freedom from excessive evaporation, 

 ri^ther than the nature of the soil. 



There is a gradual thinning out of eastern species toward the western 

 part of the State, the western limits of their ranges often extending as 

 slender tongues along the valleys of the larger rivers, a fact which seems 

 to point to the reduced moisture supply as the determining factor limi1> 

 ing their ranges. Prairie crab apple (Pyrus ioensis), Amercian hazelnut 

 (Ooryhts americana) , heech {Fagus grandifnlia) , hop hornbeam (Ostrya 

 virginiana) , cucumber tree (Magnolia acuminata), smooth alder (alnus 

 rugosa), chinquapin (Castanea pumila), willow oak (Quercus phellos), 

 water oak (Quercus nigra), are found but the division of the Carboni- 

 ferous formations where they grow terminates mostly in a zone of precipi- 

 tation between 37 and 40 inches, and since the range of most of these 

 species includes regions of unlike soil conditions, but not any territory with 

 less than 37 inches of annual rainfall, it seems to follow that the amount 

 of precipitation is the governing condition. Aside from certain plans 

 characteristic of special conditions to be described later, the delimiting 

 factor in most of our plant ranges seems to be the amount of rainfall. 



RELATION OF OUR FLORA TO THAT OF ADJACENT REGIONS. 



The examination of the ranges of 1,000 of our species chosen as repre- 

 sentatives shows the following facts concerning the relation of our flora 

 to that of adjacent regions. This number of species was chosen rather 

 than the entire number (about 1500) because the ranges of about one- 

 third of them are broken, very irregular, or of such outline, siuch as in- 

 troduced and vestigial .sipecies, as not to lend evidence in the present 

 investigation. The results are given in the number of species per hundred 

 of our range, minor fractions being omitted, and major fractions used 

 as integers. 



The flora isi divided into groups as- follows: (1) Northern flora, 

 consisting of species whose southern range does not extend farther south- 

 ward than Oklahoma (or rarely slightly farther), and divided into two 

 groups: (a) species ranging through Tipper Austral or the Carolinian 



