ASPIDIUM. 



411 



The distinguishing characters of the genus Aspidium reside partly in the 

 disposition of the sori (spore masses), which are sub-globose (almost spherical), 

 dorsal (attached to the back), and terminal on the veinlets, and partly in 

 the presence of an orbicular involucre (covering) common to all the species 

 alike, and which is fixed to the frond by the centre. 



The following are the groups of which the genus Aspidium is composed : 



1. — Cyclodium (Cy-clo'-di-um), Presl. The fronds of the plants belong- 

 ing to this small group have their veins pinnate (divided to the midrib), the 

 opposite veinlets usually joining. 



2. — Gyrtomium (Cyr-tom'-i-um), Presl. The veins in the plants of this 

 group sometimes, but not invariably, unite slightly towards the edge. 



3. — Euaspidium (Eu-as-pid'-i-um), Sclwtt. Plants with veins copiously 

 anastomosing (freely intermixed). 



4. — Polystichum (Pol-ys'-tich-um), Roth. This group, by far the most 

 important in the genus, consists of plants readily distinguished by their veins, 

 which are all free, as also by the more or less leathery texture of the fronds 

 of nearly all the species, and by the peculiar character of their teeth, which 

 are usually awned (terminating in a stiff, bristle-like appendage, which is 

 evidently a prolongation of the midnerve). This group is, for botanical 

 purposes, divided into four sections as follows : 



Section I. — Species whose fronds are only pinnatifid (divided only 

 half-way to the midrib). 



Section II. — Species with fronds once pinnate (divided to the midrib), 

 the lower pinnte (leaflets) being either entire (undivided) or pinnatifid 

 only below. 



Section III. — Species having their lower pinnae once pinnate. 

 Section IV. — Species in which the lower pinnae are more than once 

 pinnate. 



The geographical distribution of the Buckler or Shield Ferns is very 

 extensive, for many of them are found in Japan, others in the East and in 

 the West Indies, while Tropical and also North America supply us. with 

 some of the most distinct kinds belonging to the group Polystichum. Even 

 New Zealand comes in for its share of Aspidium,, and the genus is handsomely 

 represented at home by the common Prickly Shield Fern, A. (P.) aculeatum, 

 and its remarkable form, lobatum ; the latter is so thoroughly distinct that 



