CHAPTER XXII 
THE GREAT 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- 
copodiales (Club-mosses). The Ferns 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 types 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 
hundred 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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