500 General Botany 



A fifth group of plants that contributed to the forests of the 

 Carboniferous is the family of cordaites (Cordaitales). These 

 were much-branched trees, sometimes a hundred feet in height, 

 with dense foliage of parallel-veined narrow, simple leaves. 

 They had cones of two types in which the small and large spores 

 were developed, much as they are formed in the cycads. In 

 the large cones nut-like seeds were produced. The wood of 

 cordaites bears a striking resemblance to the wood of some of 

 the living conifers. 



In the vegetation of the later Paleozoic, then, there were forms 

 that combined in various ways the special characteristics of the 

 ferns, the lycopods, the equisetums, and such seed plants as the 

 cycads and conifers. The development of complex vascular 

 systems, of stems with wood and cambiums, and the develop- 

 ment of seeds were the great advances made during the Paleo- 

 zoic. 



The Paleozoic closed with an uplift of the continents, and con- 

 sequent increase of land areas, and increased drought. There 

 is also evidence of glaciation during the Permian. These were 

 doubtless important factors in the extinction of many forms of 

 Paleozoic plants. 



The vegetation of the Mesozoic. The Mesozoic era was 

 marked by the extinction of the cordaites and seed ferns and the 

 reduction of the lycopods and equisetums to herbaceous rem- 

 nants that are of slight importance in the vegetation. The 

 ferns continued their existence as forest undergrowth, but were 

 early forced to compete with a new group of seed plants, the 

 " fossil cycads " (Bennettitales). These plants had compound, 

 fern-like leaves and usually short, thick, woody trunks like those 

 of modern cycads. In most forms the reproductive structures 

 consist of a whorl of microsporophylls surrounding a central 

 cone-shaped body bearing the megasporangia. In the extreme 

 forms the inflorescence is highly suggestive of certain angiosperm 

 flowers. The forests of the Mesozoic were dominated by the 



