Chapter 1 



The angiosperms are the dominant seed-bearing plants of the present 

 day, a vast and varied assemblage estimated to consist of 300,000 species. 

 They are commonly considered the "modern" seed plants, geologically 

 young. But, structurally, they do not appear to be of recent origin; even 

 those families now considered most primitive have some well-advanced 

 characters. Evidence is accumulating from the fossil record and from 

 critical morphological studies that the angiosperms are an old group in 

 which there was early differentiation along several lines and that the 

 group had already become diverse and complex by the early Cretaceous 

 period, long considered the time of their origin. The angiosperms, in 

 their dominance, have not, however, "crowded the gymnosperms from 

 the face of the earth," as is often stated or implied in elementary text- 

 books; large areas of conifer forest exist in the tropics and in both 

 northern and southern temperate lands; the cycads are dominant in 

 some small areas in Australia and South Africa. 



In all characters, gross and minute, external and internal, sporophytic 

 and gametophytic, the angiosperms show great diversity of form, a 

 diversity that is clearly the result of adaptive specialization over a very 

 long period of time and under great climatic changes. This specializa- 

 tion leads to both increasing complexity and simplification of structure. 

 Simplicity of form has long been considered evidence of primitiveness; 

 the part played in specialization by reduction, "retrogression" or "sup- 

 pression," has been commonly overlooked. The realization that sim- 

 plicity is often secondary rather than primitive has played a prominent 

 part in the interpretations of comparative morphology and phylogeny in 

 the twentieth century. The results of reduction are prominent in the 

 morphology of every part of the plant — in the body of the sporophyte, 

 embryonic and mature, and in gametophyte origin and form. 



The angiosperms are commonly set apart from other seed plants by 

 enclosure of the seeds, in contrast with the naked seeds of the gymno- 

 sperms; by the presence of vessels in the wood; and by the possession 

 of a complex reproductive structure, the flower. But these characters do 

 not sharply limit the angiosperms. The carpel is open at the time of pol- 

 lination in some taxa, and some conifers (Araucaria) have the seed 

 enclosed; tlie vessel is present in Selaginella, Equisetum, Pteridium, 

 Ephedra, Welwitschia, and Gnetum and is absent in many angiosperms. 



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