224 EVOLUTION OF PLANTS 



evidently the remnants of a disappearing type. As we 

 have endeavored to show in a previous chapter, the 

 primitive nature of the Marattiacese is also shown by 

 the structure both of gametophyte and sporophyte. The. 

 Leptosporangiatse, which at present are the predomi- 

 nant types of ferns, are absent from the older forma- 

 tions, and first appear with certainty in the early 

 Mesozoic rocks. The earliest ones belong to the fami- 

 lies which are nearest the Eusporangiatse, while the 

 more specialized forms appear later. 



While the ferns at least the Leptosporangiates 

 are still important factors in the present vegetation of 

 the earth, the other two orders are very much less 

 prominent, and many of the types related to them are 

 now quite extinct. Of the Equisetinese, or horsetails, 

 only the genus Equisetum survives. This same genus 

 can be traced back to the Mesozoic, and possibly even to 

 the later Palaeozoic rocks, where it is associated with 

 many peculiar genera which disappear completely in the 

 later formations. Among the largest and best known 

 of these ancient forms are the species of Calamites, 

 which were like gigantic horsetails, and whose stems 

 exhibit a secondary thickening of the vascular bundles, 

 SQ. that the stem continued to increase in size until the 

 plant assumed tree-like proportions. Another character- 

 istic group was the Annulariese, a peculiar family mainly 

 restricted to the Carboniferous and sometimes associated 

 with Calamites. In the few cases where the cones of 

 these fossil Equisetinese have been preserved, they show 

 an arrangement of the tissues and sporangia much like 

 those of the existing species of Equisetum. It is evi- 

 dent that some of these ancient Equisetinese were hete- 



