62 ON GENERA AND SPECIES. 
scopic examination to discover that it ever was present; that. ` 
organ, therefore, cannot be depended upon for determining | 
the genera of the tribe Phegopteridee 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, s 
all contracted fronds with naked sporangia closely occupy- i 
ing the whole underside, or in irregular masses, are referred . 
to the tribe Acrostichee ; but in some this character is not 
always perfect. For instance, in the genus Pecilopteris, | 
the fertile fronds vary in their degree of contraction, yj 
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 Pæcilopteris 
to be with Menisciwm, This affinity is further confirmed 
on comparing it with the original herbarium specimens of 
BMeniscium simplex (Lond. 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 fn 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 
Acrostichee. Other instances also occur to show that 
Meniscium and Pecilopteris are naturally related. 
I have now said sufficient 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 Jenkinéia,: 
Hook. Gen. Fil, t. 75, B. 
