CRYPTOGAMS 353 
be slow; but the sporophyte with its millions of spores, each 
capable of producing a new individual, enables the species to 
multiply indefinitely. At the same time the interposition of 
a gametophyte, or sexual generation, secures the introduc- 
tion of a new strain with effects analogous to those of cross 
fertilization. 
411. Classification of pteridophytes.—In our study of 
this group, the ferns have been taken as the type because 
they are the most familiar and most widely 
distributed of all the vascular cryptogams. 
But while they exceed in numbers, both of 
individuals and species, all the other orders 
combined, they form only one division of thres 
great groups that make up the class Pterido- 
-phyta. These groups are: (1) ferns, under 
which are included, besides the true ferns, two 
widely differing orders, with the grape ferns 
and adder’s-tongue in one, and the water ferns 
in the other; (2) the club mosses, embracing 
the two subdivisions of Lycopodium and Sel- 
aginella; (3) the horsetail family, including 
horsetails and scouring rushes. Orders (2) 
and (8) are grouped together as cone-bearing 
(strobilaceous) pteridophytes, because their 
sporangia are clustered in oblong heads, or 
strobiles (Fig. 509), somewhat like the cones of ee 
the pine. The orders of pteridophytes differ Part of the fruit- 
‘ ing stem of a 
greatly among themselves, but agree in pos-  gcouring rush, 
sessing certain characteristics that point to Zavisetum limo- 
their derivation from a common ancestry. peter a ee 
412. Distinction between pteridophytes and oo (After 
bryophytes. —In passing from the Thallo- 
phytes and Bryophytes to the vascular cryptogams, we cross 
the widest chasm in the vegetable kingdom — a gap relatively 
as great as that between vertebrates and invertebrates among 
animals. The most important modifications that discrimi- 
