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Station, Missouri Botanical Garden, and the University of Ver- 

 mont. To the curators of these herbaria thanks are due and are 

 hereby most heartily accorded for the privilege of examining the 

 collections under their charge. 



For collections of materials representing different localities 

 and for various assistance, acknowledgments are due to Mr. 

 E. Bartholomew, Stockton, Kans.; Rev. J. M. Bates, Red Cloud, 

 Neb.; Professor E. Bethel, Denver, Colo.; Dr. J. J. Davis, Racine, 

 Wis.; Professor W. G. Farlow, Cambridge, Mass.; Dr. Ed. Fischer, 

 Bern, Switzerland; Professor F. D. Heald, Austin, Texas; Dr. G. G. 

 Hedgcock, Dr. Perley Spaulding, and Mr. Carl Hartley, Wash- 

 ington, D. C; Professor E. W. D. Holway, Minneapolis, Minn.; 

 Professor H. S. Jackson, Corvallis, Oregon; Professors M. E. Jones 

 and A. O. Garrett, Salt Lake City, Utah; Professor F. E. Lloyd, 

 Auburn, Ala.; Dr. Donald Reddick, Ithaca, N. Y.; Mr. A. B. Sey- 

 mour, Cambridge, Mass.; Dr. A. R. Sweetser, Eugene, Oregon; 

 Dr. H. Sydow, Berlin, Germany; Dr. W. Tranzschel, St. Peters- 

 burg, Russia; Dr. Roland Thaxter, Cambridge, Mass.; Professor 

 S. M. Tracy, Biloxi, Miss.; and to generous friends in various 

 parts of the country who have severally contributed specimens. 

 I am also indebted to Mr. W. W. Eggleston of the U. S. Depart- 

 ment of Agriculture, and to Mr. J. G. Jack of the Arnold Arboretum, 

 for the determination of certain host plants. 



The writer was first induced to begin a study of this group 

 chiefly because of the great need of systematic work which is at 

 once apparent to any one who attempts to determine specimens 

 either of the cedar or apple rusts. Much work has been done with 

 the group, and this fact, doubtless, has led many to suppose that 

 it was well worked and that most of the problems concerning it 

 were settled. In taking up the study of the group, the writer, 

 however, experienced much difficulty both because the data left 

 by the past workers were chaotic and especially because they 

 were incomplete. An incentive for the continuation of these 

 studies has been the possibility that a detailed study of such a 

 parasitic group, with its restricted hosts and abbreviated life- 

 cycles, might throw light on some of the broader biologic problems 

 such as the nature of relationships between hosts and parasites, 

 and the possible origin and present trend of development among 

 pleomorphic and heteroecious forms. 



The geographic range which these studies have covered has 



