6 
POLYSTICHUM ACULEATUM. 
[Acute Many-order, from the many lines of seed spots. J 
COMMON PRICKLY SHIELD FERN. 
Synonyms. — Polypodium aculeatum, Aspidium aculeatum. 
The student will experience some difficulty between 
this fern and the P. angular e. When their principal 
distinguishing characteristics are in each strongly 
marked it may be easy enough to see the differences 
when two such specimens are placed side by side. 
But when the guiding points; of the P. aculeatum are 
a little relaxed, and when those of the P. angulare 
are not well defined, the two species assimilate to 
each other so much as to blend into a sort of indis- 
tinctness. When such negative, neutral, or mediastie 
plants as these are met with — and they are the most 
commonly met with— -it becomes hard to assign them 
their true place and their true name. In the acule- 
atum the pinnules are narrower and decurrent, or 
merging into the rib of the pinna that supports 
them, but in the angulare they are attached by a 
short stalk: in the former they are of a darkish 
glaucous green, but are of a lighter colour in the 
latter : in the first, the pinnules form an acute angle 
at the point to which they are attached, whilst in 
the last, that angle is an obtuse one : and in the 
aculeatum the pinnules are somewhat convex on th© 
upper surface, whereas, in the angulare they are flat. 
These seem to be small differences; nevertheless, 
if they are constant, they are enough to establish 
two acknowledged species. As long as a difference 
is constant it is sufficient, no matter how small it 
may be. As to the rest, the frond of the P, aculeatum 
is lance-shaped, or broadly lanceolate ; texture harsh, 
but this can be only tested by comparison ; the stipes 
or lower stem enveloped in rust-coloured scales; 
pinnules with an auricle or projection on the upper 
and outer side of each, serrate, spinulose; sori abun- 
dant, distinct, small, and brown ; the indusium or 
cover is circular, which is characteristic of the genus 
Polystichum , and is fastened down by an attachment 
in the centre, as a buttdn is sewn to a coat. 
