1910.] NATURAL SCIENCES OF PHILADELPHIA. 541 



FLORA OF THE C0N0WING0 BARRENS OF SOUTHEASTERN PENNSYLVANIA 



BY FRANCIS W. PEXNELL. 



During the summers of 1908 and 1909 the writer was enabled to 

 spend considerable time in the stud}' of the flora of the Conowingo 

 or Serpentine Barrens lying in Chester and Delaware Counties, Penn- 

 sylvania. Nearly all the areas in these counties were visited, in most 

 cases repeatedly, and full collections made. The writer has also been 

 able to examine the material in the herbaria of the Academy of Natural 

 Sciences of Philadelphia, the Philadelphia Botanical Club, and the 

 University of Pennsylvania. 



The object of this paper is to present accurate data as to the species 

 composing the flora of the barrens. Effort has been made to make 

 the list of species accurate and complete, while less attention for the 

 present has been paid to the ecological grouping and adaptations of 

 the flora. The general conditions of the environment — climate, 

 physiography and soil — will, however, be considered, and a sketch 

 of the flora and its affinities presented. 



In the prosecution of this study the writer acknowledges the 

 courtesy of the custodians of the herbaria named, and especially 

 that of S. S. Van Pelt and Witmer Stone, to whose interest he is 

 indebted for much aid in the determination of species. Also to 

 Agnes Chase, of the United States Department of Agriculture, is he 

 indebted for determinations in the genus Panicum. 



Climate. 



The climate of this district, as taken from the reports of the 

 Weather Bureau stations at West Chester and Kennett Square, 

 shows the general condition prevailing throughout the Middle Atlantic 

 States. The average temperature for January is 30° F.: for July 

 74° F. ; the average first killing frost of autumn is on October 19, 

 the last of spring on April 16; the precipitation is relatively even and 

 well diffused throughout the year, reaching a total average annual 

 rainfall of 50 inches. Thus it may be seen that the growing season 

 is of considerable length, warm and well supplied with moisture. 

 The xerophytic character of the Conowingo flora is not the result 

 of the climate. 



