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 Marattiaces is also shown by 
the structure both of gametophyte and sporophyte. The 
Leptosporangiatz, 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 Eusporahgiate, 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 Equisetinez, or horsetails, 
only the genus Equisetum survives. This same genus 
can be traced back to the Mesozoic, and possibly even to 
the later Paleozoic 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, 
so that the stem continued to increase in size untjl the 
plant assumed tree-like proportions. Another character- 
istic group was the Annulariee, a peculiar family mainly 
restricted to the Carboniferous and sometimes associated 
with Calamites. In the few cases where the cones of 
these fossil Equisetinee 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 Equisetinee were hete- 
