468 MORPHOLOGY OF THE ANGIOSPERMS 



The necessity of considering the nature of simplicity in form is, of 

 course, only a part of the understanding of the entire story of evolu- 

 tionary change, but the nature of simplicity needs emphasis, because 

 its neglect has been a major element in misinterpretations. In the search 

 for a time of origin and an ancestral stock for the angiosperms, it is 

 necessary to know, as completely as possible, the story of evolutionary 

 modification of all organs and tissues. On this basis, the evolutionary 

 stage of a fossil can be interpreted, and the age of angiosperms, at 

 that tim.e, perhaps be estimated. 



Since 1920, thorough morphological studies of the Ranales and of 

 some of the more primitive monocotyledons have provided a basis for 

 an estimate of the relative advance of a taxon (Chap. 11). These 

 studies have shown that a decision as to the level of advance must rest 

 on all possible characters, not on one or two; uneven advance in struc- 

 tural specialization is common, and the sum total of modifications is 

 needed to estimate the stage of evolution. 



Assessment of the general level of morphological advance in promi- 

 nent genera of Lower and Middle Cretaceous floras should give a basis 

 for an opinion on the probable age of those floras. Popiihis, Sassafras, 

 and Plotarms — well-known Cretaceous genera of unquestioned identity 

 — give evidence that the angiosperms, as early as tlie Cretaceous were 

 already highly specialized, for the three genera show many advanced 

 characters. In general flower structure, simplicity has replaced com- 

 plexity. All are dioecious; two are anemophilous. The stamens have 

 terete filaments and specialized anthers; carpel number is reduced to 

 two in Popultis, and one in Sassafras and Platanus. The gynoecium in 

 Populus is syncarpous. The ovules are solitary in Sassafras and Platanus. 

 There is little evidence of the primitive follicle in the achene of 

 Platanus or the drupe of Sassafras. In nodal structure, no evidence re- 

 mains of the primitive two-trace structure. The xylem of all genera is 

 specialized. The angiosperms had had a long history before Cretaceous 

 times. 



By Middle Cretaceous times, the fossil record shows great diversity 

 among angiosperm families, and many families now considered basic 

 had become cosmopolitan. The Cretaceous flora of AustraHa is not 

 different in its basic families from floras of other continents. After early 

 differentiation of fundamental families, the angiosperms had spread 

 throughout the world. This widespread flora was a "matiue" flora; it can 

 hardly be considered young in the Cretaceous. 



In a discussion (1960) of the age of the angiosperms, a probable 

 paleozoic origin for these plants is questioned. But the evidence of high 

 specialization of many of the Lower and Middle Cretaceous genera is 

 not considered. Morphology of living plants strongly supports the evi- 



