FILI CINE A E. HOMOS POR US F I LI CINE A E. 



217 



from underneath the young sporangium on the margin of the leaf in the form of 

 an annular wall, which incloses the sporangium in a pocket ; the upper side of the 

 wall has the structure of the upper side of the leaf. A false indusium, which is not, 

 like those above described, a new formation from the leaf, is produced by the 

 folding or rolling back of the margin of the leaf over the sorus, as in Allosurus, 

 Cheilanthes, and many species of Pteris. 



Sori are not generally formed on all the leaves of the full-grown plant ; sometimes 

 groups of fertile leaves alternate at regular periods with groups of sterile leaves, as in 

 Slruthiopteris germanica ; sometimes the sori are 

 distributed uniformly over the whole of the sur- 

 face of a leaf, in other cases they are confined to 

 certain sections of it. The fertile leaves may be 

 in other respects similar to the sterile, or they may 

 be strikingly different from them ; the latter case 

 is often the result of the total or partial absence 

 of the mesophyll between and near the fertile 

 veins ; the fertile leaf or the fertile portion of the 

 leaf then looks like a spike or panicle of sporangia, 

 as in Osmunda and Aneimia. The sporangia are 

 usually formed on the veins of the leaf, and on 

 the under surface or on the margin of the 

 lamina; but in the Acrostichaceae they are 

 derived from the mesophyll, as well as from the 

 veins; in Olfersia they cover both surfaces of 

 the leaf by the side of the mid-rib, in Acrostichum 

 only the under surface. Where, as is usually the 

 case, the veins alone bear the sporangia, the 

 fertile veins may be similar to the sterile, or the 

 fertile undergo certain changes at the spots where 

 they bear the sori; they may swell up into the 

 form of a cushion, and so form a receptacle, or, 

 as it would be better to call it, a placenta, for the 

 receptacle in the Ferns is homologous with the 

 placenta in the Phanerogams ; or they project 

 beyond the margin of the leaf, as in the Hymeno- ^S^KS^SSXS^ " 

 phyllaceae. The sorus may be on the extremity 55 times ' 



of a vein, which then often forms two branches in the angle of which the sorus is 

 placed, or it is on the dorsal surface of the vein behind its extremity, or it runs 

 for some distance along the side of the vein ; the fertile veins sometimes run close 

 to the margin of the leaf, or they may be near the mid-rib or in some other part 

 of the lamina 1 . 



The process in the development of tht sporangium' 2 ' is essentially the same in all 



FlG . l6s . 



1 [Celakovsky, Unters. ii. d. Homolog. d. generativ. Prod. d. Fruchtbl. b. d. Phanerog. und 

 Gefasskrypt. in Pringsheim's Jahrb. XIV, 1883.] 



3 Reess, Entwickl. d. Polypod.-Sporang. (Pringsheim's Jahrb. V, 1866). Russow, Vergl. 

 Unters. Petersbourg, 1872. Prantl, Untersuch. ii. d. Gefasskrypt. I. and II. 



