I9I4.] SOUTHEASTERN NORTH AMERICA. 235 



world-wide distribution as have several families of Monocotyledonse 

 and Gamopetalse. No distinctly boreal group has been developed as 

 among the Gamopetalse (Ericales). Certain great families charac- 

 terize the north temperate region and these are all herbaceous forms 

 believed to be of relatively recent origin, e. g., Polygonaceae, Caryo- 

 phyllaceae, Cruciferse, Saxifragacese, Onagrace?e and Umbelliferse. 

 While aquatic forms are common this habit does not characterize 

 whole families as among the Monocotyledon^e. The Choripetalse 

 predominate in the American tropics and many of the families 

 present in the Wilcox flora have been shown to have probably origi- 

 nated in that region. 



The second grand division of the DicotyledoncX, the Gamopetalse 

 (Sympetalse), constitute a rather well-defined group, presumably de- 

 rived from the Choripetalse, and characterized by a complete cyclic 

 arrangement of the floral parts, a usually gamopetalous corolla, 

 ovules with a small nucellus and usually a single integument. It 

 contains nine or ten orders and upwards of 50,000 existing species. 

 The majority of the orders appear to be more compact and natural 

 groups than the corresponding alliances among the Choripetala;. 

 The Ericales, Primulales and Ebenales are pentacyclic and isocarpous, 

 while the Gentianales, Polemoniales, Personales, Plantaginales, Rubi- 

 ales, Valerianales and Campanales are tetracyclic and anisocarpic, 

 the last three orders being epigynous. 



The alliance predominates in herbaceous forms and several of 

 the families are distinctly boreal. While the Compositae, Labiat?e 

 and Plantaginacese are of world-wide distribution there are no 

 notable continental pairings such as is usually the result of an 

 extended geologic history. These and many other facts suggest that 

 the Gamopetalse as a whole, especially the more evolved, herbaceous, 

 extratropical families, are of relatively modern origin whose major 

 specific differentiation was concomitant with the occupation of the 

 temperate zones after the retreat of the Pleistocene ice-sheets. 



From the viewpoint of floral structures the so-called Compositse 

 are clearly the culmination of the evolution of floral structures. This 

 is shown not only by their gamopetaly, epigyny, connivent anthers, 

 and the formation of seedlike fruits with a pappus, but by the com- 

 plex flowerhead, the prevalence of diclinism, the dimorphism of the 



