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 three 

 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 (3) 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 

 the pine. The orders of pteridophytes differ 

 greatly among themselves, but agree in pos- 

 sessing certain characteristics that point to 

 their derivation from a common ancestry. 



412. Distinction between pteridophytes and 

 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- 



FIG. 509. 

 Part of the fruit- 

 ing stem of a 

 scouring rush, 

 Equisetum limo- 

 sum, showing the 

 cone-like spore 

 cluster. (After 

 GRAY.) 



