CHAPTER XXII 



THE GEEAT GROUPS OF PTERIDOPHYTES 



216. The great groups. — At least three independent lines 

 of Pteridophytes are recognized : (1) Filicales (Ferns), 

 (2) Equisetales (Scouring rushes, Horsetails), and (3) Ly- 

 cojpodiales (Club-mosses). The Perns are much the most 

 abundant, the Club-mosses are represented by a few hun- 

 dred forms, while the Horsetails include only about twenty- 

 five species. These three great groups are so unlike that 

 they hardly seem to belong together in the same division 

 of the plant kingdom. 



Filicales (Ferns) 



217. General characters. — The Ferns were used in the 

 preceding chapter as tyjoes of Pteridophytes, so that little 

 need be added. They well deserve to stand as types, as 

 they contain about four thousand of the four thousand five 

 Imndred species belonging to Pteridophytes. Although 

 found in considerable numbers in temperate regions, their 

 chief display is in the tropics, where they form a striking 

 and characteristic feature of the vegetation. In the trop- 

 ics not only are great masses of the low forms to be seen, 

 from those with delicate and filmy moss like leaves to those 

 with huge leaves, but also tree forms with cylindrical 

 trunks encased by the rough remnants of fallen leaves and 

 sometimes rising to a height of thirty-five to forty-five 

 feet, with a great crown of leaves fifteen to twenty feet 

 long (Fig. 297). 



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