Ch. XII] FOSSIL PTERIDOPHYTES 509 



The subject of the alternation of generations is one of the 

 most interesting in plant morphology. It has been men- 

 tioned often in the preceding pages, and may now be sum- 

 marized to advantage in a diagram (Fig. 361). It should be 

 noted, however, that not all investigators take the view of 

 its origin accepted in this book, for some maintain that the 

 sexual and asexual generations were originally not different 

 but alike in structure, as Polysiphonia suggests, the differ- 

 ences being supposed to have developed subsequently. This 

 is termed the homologous theory of alternation, as con- 

 trasted with the antithetic theory now generally accepted. 



Viewing the Pteridophytes as a whole, it is a striking fact 

 that the living forms are insignificant in number and size 

 as compared with those which flourished in past geological 

 ages, notably in the Carboniferous Period. Then there 

 were many great Tree Ferns, of the eusporangiate type, and 

 giant arborescent Horsetails, especially of the kinds called 

 Calamites, and stately tree Lycopods, of the genera Lepido- 

 dendron and Sigillaria (Fig. 362). Of these and many 

 more, the remains are found finely preserved as fossils in 

 the rocks, while they make up the chief part of the material 

 of the coal beds. These beds were formed in great swamps 

 of lush vegetation which was preserved from decay by the 

 presence of organic acids, precisely as in case of the peat 

 bogs still forming in northern regions (page 486). If one 

 asks why the Carboniferous Period in particular was the 

 time of such active coal formation, the answer is probably 

 this, that previously there was no great amount of large woody 

 swamp vegetation. The Bryophytes, probable predecessors 

 of the Pteridophytes, were only carpet plants, unable to rise 

 above the ground ; and hence they were unable to utilize the 

 greater abundance of carbon dioxide then prevailing in the 

 atmosphere. The Pteridophytes, however, with their vas- 

 cular system, were able to form great forests, which rapidly 

 fixed the abundant carbon dioxide as carbon in their tissues, 

 setting free the oxygen ; and the process continued until the 



