2 American Midland Naturalist Monograph No. 7 



when they constitute a part of the recognizable flora. Stray plants or 

 waifs, and especially those that have not been collected within the past 

 half century in Illinois, and which presumably no longer occur spon- 

 taneously, and species reported for the state for which no reliable 

 voucher specimens have been found, usually have been excluded. Un- 

 critical acceptance of all published records is not a satisfactory basis 

 for dealing with the species of a flora. 



In this edition the taxonomy has been extensively reviewed, not 

 only in the treatment of genera and species, but especially in the 

 sequence of families by breaking away from the familiar but partly 

 outmoded Englerian system which as early as 1893 was characterized 

 by C. E. Bessey as "a makeshift maintained by conservatism." The 

 stimulus of Bessey, and later of the great modern botanical phylogenist, 

 John Hutchinson of Kew, whose Families of Flowering Plants, one of 

 the most important botanical works of its kind produced in the twenti- 

 eth century, has influenced not only the study of phylogenetic botany 

 but other related fields, such as systematic plant anatomy. Although 

 we are unable to accept Hutchinson's system in its entirety, we have 

 adopted many of the principles and practices so ably expounded by 

 him. The sequence of families followed herein will be more or less 

 familiar to those botanists who are acquainted with Hutchinson's 

 British Flowering Plants, Metcalfe & Chalk's Anatomy of the Dicotyle- 

 dons, and Clapham, Tutin & Warburg, Flora of the British Isles. 



According to theories of Bentham & Hooker, Bessey, Hallier, Wie- 

 land, Arber & Parkin, Hutchinson, and their many followers. Dicotyle- 

 dons beginning with Ranales are placed first, and Monocotyledons 

 follow as a derived subclass. The orders of the Archichlamydeae of 

 Engler are extensively rearranged, but those of the Metachlamydeae, 

 considered to comprise a polyphyletic series that has reached a similar 

 evolutionary level, have been retained largely as Engler left them. 

 Monocotyledons show two evolutionary divergent lines, the first large- 

 ly entomophilous from Alismales to Orchidales, the second progressively 

 anemophilous from Arales to Graminales. 



This study is based mainly upon material contained in the herbari- 

 um of the University of Illinois, including more than 400,000 specimens 

 from various parts of the earth. Of these approximately one-fourth 

 were collected in Illinois. Directly or indirectly ten other institutional 

 herbaria in Illinois have been drawn upon, particularly Chicago Nat- 

 ural Histoiy Museum, Illinois State Natural History Survey, Southern 

 Illinois University, and Illinois State Museum, representing a total of 

 more than 150,000 Illinois collections of vascular plants. 



Acknowledgments 



I am under obligation to so many persons for assistance that it is 

 scarcely practical here to mention all, but I should like to express 

 cordial thanks to Dr. Virginius Heber Chase of Peoria Heights, Illinois, 

 for his continued collaboration, and for reference to his magnificent 

 herbarium of nearly 50,000 specimens recently purchased by the Uni- 



