FERNS, FOSSILS AND FUEL 
Among the pteridophytes are several groups which 
have completely disappeared, such as the psilophytales, 
which are known only in a long-past geologic period, and 
the sphenophyllales which also became extinct at a very 
early date. Other members of the group are the horse¬ 
tails (equisetales), which date almost from the dawn 
of known plant life on earth; the club mosses (lycopo- 
diales), which date back to a very early period; and the 
true fern group (filicales), which are also extremely old. 
The ferns are the only group which seem still to flourish, 
for the horse-tail and the club moss are represented to¬ 
day only by insignificant types which cannot compare 
with the gigantic representatives of their orders that 
grew in early geologic periods. 
The groups of plants so far discussed are all called 
cryptogams or non-flowering plants because of their 
method of reproduction which is carried on by a single 
cell or spore. The next great group is that of the sper- 
matophytes or seed plants. They reproduce sexually 
by developing a seed which is not a single cell, but a 
complex, well protected body. 
Between the cryptogams and the spermatophytes there 
existed at an early geologic age a now extinct group called 
pteridosperms. These were a combination of seed plant 
and fern. They must have looked something like the 
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