FERNS. 25 1 



(a) Acrostiche(E. The sori cover the surface and veins of the under side or of 

 both sides, or are placed upon a thickened receptacle which stands on the vein. There is 

 no indusium. (Acrostichum, Polybotrya.) 



(b) PolypodiecB. The sori occupy either the whole length of the veins, or special 

 anastomosing branches of it, or the back or thickened end of a vein. They are naked, 

 or with a lateral indusium. (Polypodium, Adiantum, Pteris.) 



(c) Asplen'iecE. The sori are unilateral on the course of the veins, and are covered 

 by a lateral indusium, or rarely without any; or they extend at their apex over the 

 back of the veins, and are covered by an indusium springing from it ; or they occupy 

 special anastomosing branches of the veins, and are unilateral and covered by an indusium 

 free on the side of the vein. The leaf-stalk is not articulated. (Blechnum, Asplenium, 

 Scolopendrium.) 



(d) Aspidiea. The sori are dorsal on the veins, covered with an indusium, or 

 terminal and without indusium. (Aspidium, Phegopteris.) 



(e) DavalliecB. The sori are terminal on a vein or at a fork, and are furnished with 

 an indusium ; or are placed on an intramarginal anastomosing bend of the veins, and 

 covered with a cup-shaped indusium, free at the outer margin. (DavaUia, Nephrolepis.) 



The Marattiace£B, hitherto included among Ferns, must, from the earlier state- 

 ments of Russow and the more recent investigations of Luerssen (Habilitations- 

 schrift, Leipzig 1872), be separated from them, and classed with the Ophioglossaceae 

 (and Equisetaceae), in consequence of the entirely different mode in which their spor- 

 angia are formed. The large sporangia of Marattia are placed singly on lateral veins 

 of the pinnae, to which they are attached by a narrow ridge-shaped base (pedicel). 

 Two longitudinal rows of loculi contain the spores, which are not formed, as in the 

 true Ferns, from a single primary mother-cell (central cell), but from a mass of tissue 

 composed of primary mother-cells, filling up the loculus. The single loculus of the 

 sporangium of Marattia corresponds so far to the single sporangium of Ophioglossum. 

 A nearer affinity to the Ophioglossaceae may be indicated by the stipular struc- 

 tures of the Marattiaceae, which, while entirely foreign to Ferns, exhibit a certain 

 resemblance to those of the Ophioglossaceae. 



[The classification of Mettenius given above will serve as a guide to the principal morpho- 

 logical differences between the various types of Fern structure. The student may however consult 

 Hooker and Baker's Synopsis Filicum (London, 1S68) for a systematic arrangement more in accord- 

 ance with our present extended knowledge of species, though still, no doubt, artificial. — Ed,] 



