340 THE PTERIDOPHYTES 



horsetails, so nearly like the living forms of Equisetum that we 

 can readily picture their appearance along the margins of swamps 

 and streams. Curiously some of the Catamites were heterospor- 

 ous, although all of the living types of the Eqiiisetinece are 

 homosporous. The ancient representatives of the club mosses 

 (Plate VIII, 3, 4) were among the largest plants of those times, 

 reaching the height of one hundred feet or more. Some of them 

 were true lycopods, and others, as Lepidodendron and Sigillaria, 

 were evidently close relatives of the club mosses. Their large 

 trunks were covered with leaves, which fell off, leaving curious, 

 diamond-shaped scars that are very conspicuous on the fossil 

 stems. The earliest seed plants arose in these ages, but they 

 were far outnumbered by the pteridophytes. They were gymno- 

 sperms of the group Cordaitece (Plate VIII, 5), but with very 

 little resemblance to any living forms. The fructifications of 

 some of these primitive forms, somewhat intermediate between 

 spermatophytes and pteridophytes, are occasionally so well pre- 

 served that we can learn something of the structure of the 

 gametophytes developed by the spores. It is possible that we 

 shall later know much more about the origin of the seed plants 

 and the seed habit from the study of these fossils. 



After the Carboniferous Age the tree ferns, horsetails, and 

 club mosses became less abundant, and gymnosperms, like the 

 cycads and conifers, increased in numbers and became the 

 dominant forest types. There was an age of cycads in a later 

 period (Jurassic), when the earth was covered with these plants 

 as far north as Greenland and the climate must have been 

 tropical from pole to pole. We know very little about the 

 earliest forms of angiosperms. They do not appear abundantly 

 as fossils until a later period (Cretaceous), after the age of 

 cycads (Jurassic), although they doubtless had their origin 

 much earlier, for many insects were present which must have 

 had the habit of feeding on pollen or nectar. 



It is clear that the horsetails and club mosses of the present 

 time are merely the remnants of this ancient flora once dominant 



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