CHAPTER X\T 

 THE PTERIDOPHYTA 



In passing from the bryophytes to the pteridophytes, which 

 inchide the ferns, club mosses, and horsetails, we cross the widest 

 gap which exists in the continuity of the plant kingdom. Inter- 

 mediate forms between the liverworts and mosses on the one hand 

 and the ferns and their allies on the other are missing, and 

 although we may suggest various connecting links and reconstruct 

 plausible evolutionary series, the transitional plants themselves 

 have long since perished and we shall probably never know just 

 how our present typical land vegetation had its origin. 



The Advance from Bryophytes to Pteridophytes. — In the 

 advance from bryophytes to pteridophytes the relative importance 

 of the two generations has been completely reversed. The 

 sporophyte is no longer an appendage of the gametophyte but is 

 now the dominant and conspicuous generation, and has attained 

 complete independence. The sexual plant is still independent, 

 too, but it is relatively small and insignificant, and from this 

 point onward throughout the vegetable kingdom it suffers a 

 steady and progressive reduction. This shift in evolutionary 

 advance from the gametophyte to the sporophyte marks the 

 completion of that great forward step in the plant kingdom 

 whereby a true land vegetation was evolved. The gametophyte 

 is primarily an aquatic or at least a moisture-loving structure. 

 Even in its highest development among the mosses, where it 

 successfully invades the dry land, it has never been able to produce 

 there a strong and vigorous vegetation. The sporophji^e, 

 however, seems early to have solved the problem presented by 

 this radical change in environment, and when we meet it in the 

 pteridophytes it has already developed a stout, branching, sub- 

 terranean axis, the root, clothed with an abundance of root-hairs 

 for absorption; large leaves presenting to the sun a relatively 

 thick layer of chlorophyll-bearing tissue, which is well provided 

 with air spaces and is protected by an epidermis in which are 

 typical stomata; and a stout stem on which the leaves are lifted 



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