546 



General Botany 



water, high temperature, and the enormous pressure of overlying 

 later strata. The occurrence of carbon in these rocks is pre- 

 sumptive evidence that plant re- 

 mains were present when they were 

 originally laid down. 



The known fossil record shows 

 that during successive periods of 

 the earth's history plant groups 

 succeeded one another and that 

 there was a gradual increase in the 

 diversity of plant forms, accom- 

 panied by progressive changes in 

 both the vegetative and repro- 

 ductive structures of the plants. 

 Modern plant structures are 

 clearly derived by further devel- 

 opment and modification of the 

 structures of plants of former geo- 

 logical periods. 



That the geological history of 

 each of the plant phyla provides positive evidence for the 

 evolving of new and more complex forms from previously existing 

 forms is clear and unmistakable. In the phyla Cordaites and 

 Pteridosperms, we have the record of the evolution, the world- 

 wide dispersal, and the decline and extinction of two great 

 plant groups. 



The trend of evolution. Not only does the geological record 

 furnish abundant proofs of evolution, but it shows the course of 

 the evolution of plants. The series of reproductive structures, 

 for example, beginning with simple sporangia on foliage leaves, 

 may be traced upward through the development of sporophylls 

 and finally to the production of flowers and seeds. The vascular 

 systems of plant stems show a progressive series of changes from 

 the primitive ferns to the modern flowering plants. The de- 



FiG. 351. Fossil imprint of a leaf of 

 a species of sassafras in rock of the 

 Cretaceous period. But few fossil angio- 

 sperms have as yet been found in rocks 

 formed earlier than the Cretaceous 

 period. 



