644 BOTANY 



and Japan. We know, however, that in Tertiary times this was a 

 common genus all over the northern hemisphere. The Sequoias of 

 California, and the Cypresses (Taxodium) of the southern swamps, 

 are also disappearing remnants of widespread Tertiary genera. 



Flora of the United States 



The great extent of territory within the United States, as well as 

 the great diversity of surface, is accompanied by a corresponding 

 variety in the flora, the distribution of which offers many interesting 

 problems, and illustrates nearly all types of vegetation. 



The unbroken central plains possess a continental climate of the 

 most pronounced character, with a rapid decrease in rainfall west- 

 ward. On the eastern seacoast there is abundant rainfall, increas- 

 ing southward, while on the Pacific slope the reverse is the case. 

 Finally, the arid central and southwestern plateaus are genuine des- 

 erts, whose scanty flora is extremely characteristic. The direct con- 

 nection with the Tropics results in a strong infusion of tropical 

 types in our southern flora, and the unbroken chain of mountains 

 on the Pacific slope has been an important highway for the south- 

 ward emigration of many northern types, including a few Asiatic 

 forms ; and at the east the Appalachian Mountains have also served 

 to extend the southward range of many northern species. 



Leaving aside the aquatic and strand floras, we may divide the 

 flora of the United States, roughly, into a forest flora, a prairie flora, 

 and a desert flora, these merging, more or less gradually, into each 

 other in some cases. 



The Eastern Forest. The greater part of the country east of the 

 Mississippi was originally covered with a dense forest, containing an 

 extraordinarily large variety of deciduous trees, as well as a number 

 of coniferous ones. This forest, on the extreme north, merges into 

 the subpolar forest zone, which extends more or less completely 

 across the whole continent, but hardly reaches the confines of the 

 United States. Oar northern forests, such as those of northern 

 Michigan and Wisconsin, are characterized by the frequent occur- 

 rence of the White-pine (Pinus strobus), as well as Hemlocks, Spruces, 

 and sometimes the Norway-pine (Pinus resinosd). Growing with 

 these, however, are various " hardwood " trees, especially Sugar- 

 maples, Beeches, Oaks, etc. Where the Pines predominate, there is 

 little underwood, and the ground is thinly carpeted with Club-mosses, 

 Wintergreen, and other, mostly evergreen, trailing plants. 



Somewhat farther south, reaching its finest development in the 

 Ohio Valley and along the Alleghany Mountains, is the great forest 

 of deciduous trees, with relatively few Conifers, which are often 

 entirely absent. Here the variety of trees is far greater than in the 



