PTERIDOPHYTES 113 



The strobilus is not a feature of all Pteridophytes, not appear- 

 ing among the Ferns, but it is a structure begun by the group. 

 A third important fact is the appearance of heterospory, 

 for this is the precursor of the seed. This means a differentia- 

 tion of the spores into two kinds, one kind (the smaller ones) 

 producing the male gametophytes, the other kind (the larger 

 ones) producing the female gametophytes. Another accom- 

 paniment of heterospory is that the gametophytes are de- 

 pendent and so small that they remain within the spores 

 that produce them. 



