WILLDENOW’S FERN. 
179 
tinctive, are liable to be so extremely modified by different de- 
grees of altitude, moisture, light, exposure &c. of situation, that 
an unpractised eye would often be quite unable to determine 
the species of these closely-allied plants, whence, questionless, 
has arisen the doubt as to their separate specific individuality. 
Thus, Polystichum lohatum, upon an elevated situation, pos- 
sesses a lanceolate frond, generally very close and compact ; its 
pinnae overlapping each other : occasionally, however, these are 
distant from each other to almost the extent of their width, and 
the pinnulse are more separated, so that the plant much resem- 
bles P. aculeatum. But P. lohatum, when growing in a low 
situation, is still more like P. aculeatum ; its fronds, instead of 
being lanceolate, inclining more to ovate ; its pinnulae also are 
not merely serrated, but become slightly pinnatifid ; indeed the 
plant can with difficulty be identified. These facts, and several 
others which might be adduced, show the slight value, in this 
genus, of characters derived from the outline of the frond or of 
the pinnae and pinnulae, which are all so liable to vary in this 
particular : it was, therefore, very desirable to endeavour to find 
some more constant character, by which these kindred species, 
if they proved species, might be distinguished one from the 
other ; and after many hours spent in diligent examination of a 
great number of fronds, the author discovered a difference of ve- 
nation in the two species, which he thus describes. 
“ ‘ In examining the fronds of P. aculeatum, it may be no- 
ticed that the veins which bear thecae are not continued, like the 
j rest of the vei^s, to the edge of the pinnulae, but each terminates 
I either at its mass of thecae, or at a very little distance beyond it. 
j The same thing is not observable in P. lobatum (when mature), 
for the corresponding veins in this are each continued through 
its mass of thecae to the very edge of the pinnule, and even in 
|| the fronds of immature plants of this species, when there is but 
I little fructification, the same character is perceptible in the pin- 
nulae nearest the base of the pinnae and of the frond — - the parts 
which in ferns exhibit most strikingly all the characters of ma- 
turity : in a few of the terminal pinnulae some of these veins do 
not reach the edge. It must not be concealed that in P. acule- 
atum, in an extremely few instances (being just what might be 
n2 
