478 



General Botany 



H. N. Whilford 



Fig. 293. A large tropical fern {Maratlia), with leaves 15 feet in length. 



The appearance of a vascular system in the evolution of the 

 plant kingdom may be compared with the coming in of a back- 

 bone in the evolution of animals. There could be no large land 

 plants raised far above the soil without efhcient conductive 

 tissues through which water and food may move rapidly. The 

 vascular conductive system is therefore a most important ad- 

 justment to land conditions. 



The Pteridophytes are at once the simplest of the land plants, 

 with true roots, stems, and leaves, and the most highly organized 

 plants without seeds. The origin of all these phyla is unknown, 

 and although there have been many evolutionary developments 

 since Paleozoic times, the distinctive features of each of the phyla 

 are found in the oldest known fossil forms. 



THE FERNS (fILICALES) 



The ferns attain their greatest size, number, and variety 

 in the moist tropical and subtropical regions. Some of the smaller 

 epiphytic forms, the filmy ferns, have leaf blades only a few cell 

 layers in thickness and are confined to the dripping forests of the 



