CHAPTER XIII. 



CLASSIFICATION OF THE PTERIDOFHYTES. 



THERE are three well-marked classes of the Pteridophytes : 

 the ferns (Filicince)', horse-tails (Equisetince); and the club 

 mosses (Lycopodince). 



CLASS I. FERNS (FilicinGe). 



The ferns constitute by far the greater number of pterido- 

 phytes, and their general structure corresponds with that of 

 the maiden-hair fern described. There are three orders, of 

 which two, the true ferns (Filices) and the adder-tongues 

 (Ophioglossacece) j are represented in the United States. A 

 third order, intermediate in some respects between these two, 

 and called the ringless ferns (Marattiacece) , has no representa- 

 tives within our territory. 



The classification is at present based largely upon the char- 

 acters of the sporophyte, the sexual plants being still very 

 imperfectly known in many forms. 



The adder-tongues (Ophioglossacece) are mostly plants of 

 rather small size, ranging from about ten to fifty centimetres 

 in height. There are two genera in the United States, the 

 true adder-tongues (OpMoglossum) and the grape ferns (Botry- 

 chium). They send up but one leaf each year, and this in 

 fruiting specimens (Fig. 70, A) is divided into two portions, 

 the spore bearing (x) and the green vegetative part. In Botry- 

 chium the leaves are more or less deeply divided, and the 

 sporangia distinct (Fig. 71, -B). In Ophioglossum the sterile 

 division of the leaf is usually smooth and undivided, and the 

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