FILICES 



71 



with a cell-wall of its own, and develops by cell-division into the 

 embryo, from which springs the young sporophyte, commonly known as 

 the fern-plant. The first division-wall in the oosperm is always nearly 

 vertical ; and two others follow, at right angles to it and to eacji other, 

 dividing the oosperm into octants. From the anterior of the two original 

 halves are derived the growing point of the stetn, and the cotyledon or first 

 leaf; from the posterior half the_;&^^by which it is attached to the prothal- 

 lium, and the first root. Until the differentiation of the first leaf and the' 

 fixed root, the embryo remains imbedded in the surrounding tissue of 

 the prothallium, which grows with its growth. The primary root is- 

 always small ; in the Hymenophyllacese it disappears early, and in many 



Fig. 49. — Aspleniutn decussat-um ; adventitious bud, k, 

 already rooting (natural size). 



Fig. 50. — Young sporophyte of 

 Adiantum capillus-Veneris still 

 attached to prothallium p ; ^, 

 first leaf ; w', w", ist and _2nd 

 root ; ht rhizoids of prothallium 

 (X30). (After Goebel.) 



species of Trichomanes no subsequent roots are formed, their place 

 being supplied by underground branches. 



The mature fern varies in size from that of the 'filmy ferns,' species 

 of Hymenophyllum not above an inch in height, with delicate moss- 

 like habit, to the stately ' tree-ferns ' of the Southern Hemisphere 

 (Cyatheaceae and Dicksonia, L'Herit.), fifty or sixty feet in height. The 

 stem is either ascending and vertical, or creeping on or beneath the 

 surface of the soil, or occasionally scandent (Lygodium, Sw.), often very 

 short with undeveloped internodes, and the leaves so crowded that fre- 

 quently no portion of the stem — then often called a caudex — remains 

 exposed ; while in the creeping and climbing species the leaves are often 



