54 FERNS : BRITISH AND FOREIGN. 



the true indusia, which in these cases are always attached on the 

 interior side of the receptacles, and the two combined indusia form 

 continuous or interrupted grooves, or urceolate, bilabiate, or tubulose 

 cysts, open exteriorly and containing the sporangia (plate I. figs. 

 10 and 11). 



Universal indusia occur in cases when the segments of the fertile 

 fronds are contracted. They consist simply of the margins of the 

 segments being more or less changed in texture, and rolled inwards 

 so as to include all the sori upon the segment (plate I. fig. 12). 



There is also another kind of indusium, called " indusoid scales ;" 

 they only occur in a few species of the division Eremobrya. In 

 Pleopeltis this consists of orbicular, peltate, glistening imbricate 

 disks, covering the sporangia ; in Hymenolepis they are very thin 

 and membranous ; in Schellolepis they are very irregular in form, 

 and seem to be imperfect sporangia ; their deformity being caused 

 by the excessively crowded immersed sporangia ; they are also found 

 in Tcenitis and Vittaria, and have received the name of paraphyses. 

 I however do not use this term in describing those genera. The 

 orbicular disks of Pleopeltis, however, seem to be more special 

 organs, particularly in the smooth-fronded species. 



I liave now explained the terms of the chief organs 

 and structure of Ferns made use of for their classifi- 

 cation. I fear a beginner will say it is quite enough to 

 deter any one from entering upon the study of Ferns ; 

 but he should bear in mind that it is quite as impossible 

 to read a language without first learning the alphabet 

 as to understand botanical descriptions without first 

 mastering the technical terms employed in them. 

 He will be further impressed with the difficulty of 

 study when he finds that the very first point of inves- 

 tigation is to determine whether the fern before him 

 has or has not a ring to its spore-cases. He presumes 

 that a microscope is required to determine this first 

 starting-point ; but such is not actually the case, for 

 with the aid of a pocket lens he will be able to detect 

 the presence or absence of a ring, and as annulate 



