PREFACE. Vll 



group is very unsatisfactory and that, when unusual 

 forms of some species are in question, the keys will be 

 useless ; but she feels quite unable to make any improve- 

 ment in these respects. 



The localities of species, as given in the text, were 

 compiled in the main by the two Wests, but a number 

 of others have been added by the author from recent 

 papers. The Canadian records were obtained from a 

 hitherto unpublished list kindly sent by Mr. C. W. Lowe. 



To the Department of Scientific and Industrial 

 Research at London the writer is indebted for a grant 

 which enabled her to spend an uninterrupted year at 

 the work, and she is also indebted to Dr. G. T. Moore 

 who allowed her to complete it during the tenure of a 

 Fellowship at the Missouri Botanical Garden. To 

 Professor Yapp, Dr. Jessie S. Bayliss-Elliott, and others 

 in the University of Birmingham, and to all those who 

 by their constant encouragement and interest have 

 stimulated the continuation of the work, the writer is 

 very grateful, and hereby expresses her warmest thanks. 



NELLIE CARTER. 



The Missouri Botanical Garden ; 

 August, 1922. 



