414 



THE CHANGING GENERATIONS 



THE RISE OF MODERN FLORAS 



In the history of vertebrate animals the Mesozoic era was the age of 

 reptiles, while mammals dominated the Cenozoic. Among plants and 

 many invertebrate animal groups there was no such striking change at 

 the end of the Mesozoic; the significant advances occurred at other times. 

 Following the replacement of the Paleozoic spore bearers by seed plants 

 in the Permian, three major events in the history of plants stand forth: 

 the rise of the cycads and conifers in the first half of the Mesozoic era; 



the replacement of these by flower- 

 ing plants in the Cretaceous period 

 and early Cenozoic; and the rise 

 and spread of herbaceous seed 

 plants and of grasses in middle and 

 late Cenozoic time. 



The evergreen forests of the 

 early Mesozoic. In the Triassic 

 period the lands, drained and ex- 

 tended by the Permian uplift, were 

 in many regions semiarid with 

 strongly seasonal climates. Most of 

 the plants belonged to groups which 

 still survive. Ginkgoes, conifers and 

 cycads were dominant; there were 

 some tree ferns, and many small 

 ferns and lycopods in the under- 

 growth. As a whole the flora was 

 not luxuriant and had the aspect 

 of scrub. Water was scarce, and 

 extensive root systems developed 

 for the first time. There were no flowering plants, and in the absence of 

 grasses and weeds the vast plains which today would be prairie or steppe 

 were then probably barren wastes. By Jurassic times the lands had become 

 lower and climates more humid; plant life increased in abundance but 

 continued to be dominated by the cycads and conifers. 



The palmlike cycads (Figs. 25.2 and 26.14) were numerous and varied, 

 though today they are represented only by a few surviving genera (Fig. 

 A. 19). They had thick trunks with a crown of large feather-shaped leaves; 

 some bore their naked seeds in flowerlike structures, others in cones. They 

 grew chiefly along the streams and on moist slopes. Their distant rela- 

 tives, the ginkgoes, were also represented by various genera and species, 

 though today they have but a single survivor, the curious maidenhair 

 tree (Ginkgo biloba). The ginkgoes and cycads probably arose inde- 



Fig. 26.14. A modern cycad, Cycas revoluta. 

 (Redrawn from Strassburger, Textbook of 

 Botany, by permission The Macmillan 

 Company.) 



