186 



FLOWERLESS PLANTS 



4^ 





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i-'M W^[ 



and seated marginally: in Davallia, Trichomanes, and Hymeno- 

 pliijllmn; — in continuous lines on each side of the mid-rib and 



covered: in BlecJi- 



* 



nimi; — linear and 

 scattered over the 

 under surface and 

 covered: in Asple- 

 idum and At/iy- 

 rium ; — globose, 

 mostly on the back 

 of leaf-nerves, 

 with or without 

 a cover: in Aspl- 

 dium and Nepliro- 

 diuni; — steril e 

 and fertile fronds 

 are distinct: in 

 Acrostic hum; — 

 the fruit-clusters 

 cover only the 

 upper ])arts of 

 fertile fronds: in 

 Osmnuda, Ci/a- 

 tliea'd n d A Isop/i I la 

 are arborescent. 

 Much richer 

 in Ferns was the 

 earth in ages gone 

 by, when many 

 Ferns grew as large as trees, the atmosphere being then much 

 damper and warmer than now. The falling stems of such tree- 

 ferns were floated together by mighty streams, carried away to 

 the sea, ;iiid l)iirird under sand and mud. Tlie remains of these 

 plants, tlius being shut out from the air. could not rot, l)iit were 

 slowly changed into coal. The impressions or casts of leaves 

 and stems of Ferns can be di.stiii,i;uis1i('d in many pieces of coal 

 even now (see tig. 171j. 



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Fig. \~0. — Polypodium (juercifuliuin, epiplivtical 

 on the branch of a tree. 



