540 A TEXTBOOK OF BOTANY [Ch. XIII 



apparently much in excess of any demands of utility, and 

 hence requiring some other than an adaptive explanation. 



Ecologically the Dicotyledons are the dominant terrestrial 

 mesophytes, though in general they occupy somewhat drier 

 and cooler situations than the Monocotyledons. They com- 

 prise trees, shrubs, and herbs, in somewhat equivalent pro- 

 portions. By virtue of their superior and more plastic or- 

 ganization, they have sent representatives into nearly all 

 situations where plants can grow at all, and in those places 

 are steadily displacing the older groups. It is notable that 

 while the lower forms are largely trees, the higher are mostly 

 herbs ; and the resultant implication that the Angiospermous 

 trees preceded the herbs in evolution is sustained by other 

 evidence. Their relations to the lower forms, especially the 

 Gymnosperms, have already been noted (page 527), and to the 

 Monocotyledons they apparently stand as the parent stock 

 (page 528). 



The orders of the Dicotyledons, like the Monocotyledons, 

 form an ascending series with numerous side branches but 

 no marked breaks. Accordingly we can best subdivide them, 

 somewhat arbitrarily, into the three series : 



Series A. The Primitive Dicotyledons. 

 Series B. The Differentiated Dicotyledons. 

 Series C. The Specialized Dicotyledons. 



Series A. The Primitive Dicotyledons. 

 (ArchichlamydeaB ; Apetalse.) 



These have the flowers in clusters, commonly bisexual, 

 with no petals or sepals, or only primitive ones, of indefi- 

 nite number and spirally arranged. They are largely polli- 

 nated and disseminated by wind, though in some of the 

 higher forms animals are utilized. They include some 

 twelve principal orders. 



Order 1. Verticillales : the Casuarinas. Only 

 some 20 species, of Australasian branching trees and shrubs, 



