The Evolution of Plants 



321 



193. Fossil imprint of a leaf of a 



plants appeared and how the group became more widely dis- 

 tributed and more highly diversified in form and structure. 

 Some of these ancient plant 

 groups that at one time formed 

 a considerable part of the earth's 

 vegetation have entirely disap- 

 peared ; others are now repre- 

 sented by only a few species. 



These facts all lead to the 

 conclusion that existing plants 

 have been derived from those 

 of the past. The flowering 

 plants are the culmination of 

 a long series of constructive 

 changes in plants, which en- pj^, 



abled them to live in a greater spedes of sassafras in rock of the Cre- 



• J, r • , -J- taceous period. Tke Cretaceous rocks 



■^ * were formed much later than the Car- 



Cl^asingly complex and efficient boniferous rocks, and it is only in these 



Structures for vegetative Hfe and 'f '^ '°'^ that the fossil remains of 



° Angiosperms are found. 



reproduction were developed, 



and these structures made the plants better able to endure 

 imiavorable conditions. The progressive changes in plant 

 hfe have occurred during a geological history extending through 

 many milhons of years, and it now seems impossible to account 

 for the geological record except on the basis of evolution. 

 Certainly no one has suggested any other plausible way to 

 explain the long series of gradually changing fossil forms that 

 begins with simple fernlike plants and ends in plants like those 

 fotmd on the earth today. 



Geographic distribution and evolution. When the geo- 

 graphic distribution of plant families is studied, it becomes 



