SPERMATOPHYTES 115 



as a medium), and for establishing the alternation of genera- 

 tions. 



Pteridophytes. This group stands for the development 

 of the vascular system (with its associated leaves and roots), 

 for the introduction of the strobilus, and for the beginning 

 of heterospory. 



Spermatophytes. This group stands for the development 

 of the seed and for the evolution of the flower. 



66. The two great groups of Seed-plants. The Seed- 

 plants are the most conspicuous plants to-day, for they make 

 up nearly all the vegetation that one sees. They are cer- 

 tainly more important than the other groups, not only in 

 prominence and in numbers, but also in the use made of 

 them. They are so prominent and useful that they were 

 once thought to be the only group worth studying ; but it 

 is known now that Seed-plants can be understood best by 

 allowing the other groups to explain them. 



The Seed-plants have developed as two great groups : 

 (1) those in which the seeds are exposed, and (2) those in which 

 the seeds are inclosed. The first group is named Gymno- 

 sperms (" naked seeds"), and the second is named Angio- 

 sperms (" inclosed se*eds "). The Gymnosperms are the 

 ancient Seed-plants, and are now much less numerous than 

 the more modern Angiosperms. It is the Gymnosperms, 

 therefore, that developed the first seeds and that must be 

 considered first. 



67. The ancient Gymnosperms. In most ancient times 

 in which we have plant records, when the coal was being 

 formed, and there were tree Club-mosses and tree Equise- 

 tums, the oldest Gymnosperms lived. They were very 

 abundant, for their leaves are found everywhere in the rocks 

 about the coal mines. The leaves resemble those of Ferns 

 .so exactly that they were thought to belong to Ferns, but 

 recently it was discovered that they bore seeds, and therefore 

 they are Gymnosperms. The first Seed-plants, therefore, 



