PHYLOGENY OF THE ANGIOSPERMS 467 



closed carpel was not a characteristic of the earliest angiosperms. But 

 carpel closure, like the development of the filament-anther type of 

 stamen, is clearly the result of a general trend in specialization. The 

 microsporangiate characters — four sporangia in two pairs — seem to be 

 a basic and most important character that became fixed very early in 

 the angiosperm line. 



The strongest evidence in support of the theory of monophyletic 

 origin is the common possession of the 8-nucleate embryo sac and 

 endosperm of "secondary nature" — formed by the union of a male 

 nucleus and two female nuclei. Other supporting evidence is seen in 

 the similarity in basic structure of the flowers — position, arrangement, 

 and type of appendages, which are modified in similar ways: in the 

 stamens, throughout, with four sporangia in two pairs, one pair on 

 each side of the midvein; in the follicular carpel and its anatropous 

 ovules, the primitive type in both lines; and in similar pollen, simple 

 and monocolpate in primitive families of both monocotyledons and 

 dicotyledons. 



Age of the Angiosperms" 



In the middle of the twentieth century, the opinion that the angio- 

 sperms arose in a fairly recent geologic age began to be replaced 

 by the opinion tliat the origin was one much older than the long- 

 accepted Lower Cretaceous. The basis for this change is the gradual 

 accumulation of evidence of greater age obtained largely in two fields 

 of botany. Evidence from paleobotany in the discovery of angiosperm 

 fossils in the Jurassic and Triassic has naturally been most convincing; 

 and that from morphology, although less readily recognized, has be- 

 come impressive, as the more primitive living taxa have been broadly 

 and critically studied. 



Evolutionary change in form has been, first of all, toward increasing 

 complexity. Simplicity has been commonly interpreted as evidence of 

 primitiveness, but the importance of retrogressive change — reduction 

 after complexity — is now recognized as prominent in many taxa. Sim- 

 plicity must be critically examined; it is as likely to represent high 

 specialization as primitiveness; more likely, in advanced taxa. Changes 

 in the interpretation of simple form have brought about major changes 

 in views of the phylogenetic relationships of some taxa and, therefore, 

 of their probable age. The Amentiferae are no longer considered ex- 

 amples of the earliest angiosperms and indicators of the time of origin 

 of the angiosperms. 



* This discussion contains extracts from an invitation paper presented by the 

 author at the Ninth International Botanical Congress, Montreal, 1959, entitled "The 

 Morphological Basis for a Paleozoic Origin of the Angiospenns." 



