422 PROCEEDINGS OF THE ACADEMY OF [July, 



SmalF enumerates 617 genera and 1464 species of Lancaster County 

 plants, SO that we have a generic coefficient of 42.1 per cent. 



As a general rule, the smaller the area of land, the more simple 

 and uniform the configuration of that country. There are of course 

 exceptions to this fact. Fortunately, we have two floras from which 

 we can draw conclusions. Daniels^ has studied the flora of Columbia, 

 Missouri, and vicinity and Mackenzie^ has enumerated the plants 

 of Jackson County, the same State. Columbia is in the tension belt 

 between forest and prairie. The prairie vegetation is that of Illinois 

 and Iowa; the forest vegetation is that of the Ozark Plateau of 

 Missouri and northern Arkansas. The bottoms of the Missouri 

 River bring hither the alluvial flora and in the ponds and marshes 

 occurs the hydrophytic flora of the eastern United States. Jackson 

 County is bounded on the north by the Missouri River. There are 

 river sand bars, high and rocky bluffs, and an uneven country 

 threaded by small rivers. Prairie country lies between the streams. 

 Barrens are found where the limestone rocks are covered by a thin 

 soil, with a bog region found along the Missouri bluffs west of Sibley. 

 Daniels lists 435 genera and 1058 species of plants about Columbia. 

 The generic coefficient is 41.1 per cent. Mackenzie enumerates 

 500 genera and 1141 species in the flora of Jackson County with a 

 calculated generic coefficient of 43.8 per cent. 



The Pacific coast flora also illustrates the same principle first 

 ■enunciated by Jaccard,^** that the generic coefficient is inversely 

 proportional to the diversity of the ecologic conditions. HalP^ has 

 written one of the best local floras extant. The Yosemite National 

 Park presents a great variety of conditions, but as compared with 

 the central Rocky Mountain region or the entire Sierra Nevada 

 chain its topography is less diversified. Here, however, we find 

 springs, creeks, rivers, ponds, lakes and glaciers with rocky outcrops, 

 gravelly ridges and steep precipices. The irregular topography 

 gives rise to southward-facing slopes which receive the full efifect 

 of the sun's rays, as well as northward slopes, where it is moist and 

 shady. The altitude ranges from 2500 feet to 13,090 feet along the 

 crest of the Sierra Nevada. If an enumeration is made of the 



'Small, John K.: Flora of Lancaster County, Pa., 1913. 



8 Daniels, Francis P. : The Flora of Columbia, Missouri, and Vicinity, 1907. 



^Mackenzie, Kenneth: Manual of the Flora of Jackson County, Mi.ssouri, 

 1902. 



">Jaccard, Paul: Nouvelles Recherches sur la Distribution Florale. BulLSoc. 

 Vaud. des Sci. Nat., XLIV, 259, 1908. 



■" Hall, Harvey and Charlotte C: A Yosemite Flora, 1912. 



