8 INTRODUCTION. 



of a roundish shape; which are them- 

 selves encircled (except in the instances of 

 Osmunda, Botrychium, and Ophioglossum, 

 where it is wanting) by a jointed ring, hori- 

 zontal in Trichomanes and Hymenophyllum, 

 and vertical in the rest ; the elasticity of which 

 eventually bursts open the thecse, and scatters 

 the spores when mature. The links of this 

 ring, and its operation in breaking, may be 

 plainly seen through the lens above recom- 

 mended. These thecae are, in the majority of 

 the genera, arranged on the back of the pin- 

 nules, in linear, oblong, or circular, clusters, 

 called Sori ; either having, or not having, above 

 the mass, a thin skin-like integument Indu- 

 sium nearly of the same figure as that mass 

 itself: at first, covering and inclosing the young 

 seed-cases ; afterwards torn away at its margin, 

 or cast off". In some instances, however, the 

 plant itself is moreover divided into barren and 

 fertile fronds, either of a distinctly different, or 

 of the same, form, only that the fertile are 

 rather narrower and less expanded, and come 

 out later. They also, and the fructification 

 otherwise, sometimes do not make their ap- 

 pearance at all after transplantation, until the 



