102 BOTANY. 



209. Most plants are short-lived. By far the greater 

 number perish in a year or two, as is the case with our 

 annuals and biennials. Some shrubs and trees may live 

 for a considerable number of years, but even the most en- 

 during generally die in a few centuries. The plants of the 

 world are thus constantly dying off, and are as constantly 

 being renewed. Occasionally the dying off in a particular 

 species was more rapid than the renewal, in which case the 

 species eventually became extinct: many such cases are 

 known to palaeontologists. On the other hand, it has fre- 

 quently happened that new forms have appeared as the 

 older ones have died off, so that the character of a particu- 

 lar flora has thereby been gradually changed. 



210. By a study of the fossil plants of any period in the 

 world's history we may learn that the flora of each region 

 has undergone great changes. The flora of North America 

 in the Tertiary period was very different from what it is 

 now, while the Cretaceous flora was still more unlike that 

 of the present. Plants that npw are confined to the east- 

 em continent were then comipon in many parts of this 

 continent, and tropical or sub-tropical species flourished 

 abundantly in Nebraska and Dakota. 



211. Moreover, we learn by such a study that many of 

 "the plants of the present were not yet in existence in cer- 

 tain geological periods. As we go back in geological time 

 the vegetation is less and less like that of to-day. Thus, 

 the higher flowering plaints (Dicotyledons) were not in ex- 

 istence earlier than the Cretaceous period, while the Lilies 

 and their relatives date back to the Triassic. The great 

 Carboniferous vegetation, from which our coal was derived, 

 contained no plants with true flowers. There were no 

 grasses or sedges, no lUies or orchids, no roses or violets, no 



