62 ON GENERA AND SPECIES. 



scopic examination to discover that it ever was present ; that 

 organ, therefore, cannot be depended upon for determining 

 tlie genera of the tribe Phegopteridece by herbarium speci- 

 mens. 



The relation of one genus to another is also governed by 

 the fertile frond being more or less contracted ; for instance, 

 all contracted fronds with naked sporangia closely oocupj^- 

 ing the whole underside, or in irregular masses, are referred 

 to the tribe Acrosliclme ; but in some this character is not 

 always perfect. For instance, in the g-enus Pcccilopieris, 

 the fertile fronds vary in their degree of contraction, 

 and some being but slightly contracted, the veins being 

 distinct, forming- obvious well-defined receptacles,* which, 

 when viewed in conjunction with the general habit and 

 mode of venation, indicates the relationship of Pcecilopterin 

 to be with Menisciwii. This affinity is further confirmed 

 on comparing it with the original herbarium specimens of 

 Meniscium simplex (Lend. Journ. Bot. v. 1, t. 11), which 

 are strictly characteristic of the genus Meniscium, but since 

 the species was first described it has been introduced i*u a 

 living state to the garden collection, and has produced 

 fertile fronds so completely contracted, that if such state 

 had been in the first instance observed by the learned 

 author, he would not have hesitated in referring- it to 

 Acrosiiclien. Other instances also occur to show that 

 Meniscium and Fcpcilopteris are naturally related. 



I have now said sufEcient to show that the placing of 

 such examples of naturally allied species in different genera 

 must be admitted as only an artificial convenience for 

 arrangement. 



The various forms and direction of the sori also consti- 



* On this state Sir William Hooker founded the genus Jcnlinsin. — 

 Hook. Gen. Fit, t. 75, E. 



