The Evolution of Plants 



321 



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 eartli'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 deri\Td from those 

 of the past. The flowering 

 plants are the culmination of 



a long series of constructive 



changes in plants, which en- Fig. 193. Fossil imprint of a leaf of a 



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



• . e • ^ T taceous period. The Cretaceous rocks 



variety of environments. In- ^.^^^ ^^^^^^ ^^^^ later than the Car- 



Creasingly complex and eflicient boniferous rocks, and it is only in these 

 , , r i. 4.' Tc J later rocks that the fossil remains of 



structures for vegetative life and . . . , 



° Angiosperms are found. 



reproduction were developed, 



and these structures made the plants better able to endure 

 unfavorable conditions. The progressive changes in plant 

 life have occurred during a geological history extending through 

 many millions 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 fomis that 

 begins with simple fernlike plants and ends in plants like those 

 found on the earth toda}\ 



Geographic distribution and evolution. When the geo- 

 graphic distribution of plant families is studied, it becomes 



