THE NORTH AMERICAN SPECIES OF CENCHRUS. 



By Agnes Chase. 



INTRODUCTION. 



The sandburs, common and troublesome to man and stock in sandy 

 regions throughout the warmer parts of the United States and south- 

 ward, form a compact genus of closely related species and are the 

 most highly specialized group of the tribe Paniceae. While these 

 grasses, by reason of their aggressiveness, are familiar to all in the 

 regions they inhabit, the species have been much confused. The 

 revision here offered is based primarily upon the collections in the 

 United States National Herbarium. Type specimens have been ex- 

 amined in the herbaria of the New York Botanical Garden, the 

 Academy of Natural Sciences, Philadelphia, and the Charleston 

 Museum. In 1907 A. S. Hitchcock visited the more important her- 

 baria of Europe, making notes upon the type specimens of species 

 based on American collections, 1 and taking photographs of them. 

 While his work was primarily on the genus Panicum, his study 

 of Linnaeus's and Grisebach's types included those of Cenchims. 

 For the loan of Fournier's types I am indebted to the herbarium of 

 the Universitetets Botaniske Have, Copenhagen, and to that of the 

 Museum d'Histoire Naturelle, Jardin des Plantes, Paris, and for 

 Sprengel's type to Dr. Urban of Berlin. Of some species the type 

 specimens have not been seen. In such cases the fact is stated. 



In this revision the method of work outlined in the Revision of 

 the North American Species of Panicum 2 has been followed. 



The text figures, drawn by the author, illustrate the outer face of 

 bur, that is, the side in view when the bur is attached to the axis, two 

 views of the spikelet, and one of the fruit. The figures are all magni- 

 fied five diameters. In each case the specimen from which the draw- 

 ings were made is indicated. The burs are variable, and the spikelets 

 are often distorted by the pressure of the involucre. The burs and 

 spikelets selected were as representative of the respective species as 

 possible. The spikelets are not always from the bur figured, but in 

 every case they are from the same plant. 



1 See Hitchcock, Types of American Grasses, Oontr. U. S. Nat. Herb. 13: 

 113-158. 1908; and Hitchcock and Chase, op. cit. 15: 2-4. 1910. 

 'Hitchcock & Chase, Contr. U. S. Nat. Herb. 15: 1-8. 1910. 



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