48 AMERICAN FERN JOURNAL 
tically intolerant of acid. The contrast between these 
two species will receive further attention later on. 
Polypodium vulgare is difficult to characterize, as its 
habitats exhibit a wide range in both rock and soil char- 
acter.- It appears to“be definitely limited however, on 
the acid side, failing to grow in mediacid soils; and, as 
it does grow in alkaline humus, even though infrequent 
on bare limestone rocks, it is classed as a calcareous soil 
plant rather tolerant of acidity. Polypodium polypo-— 
dioides (incanum) belongs, however, to the other class. 
The reaction of the humus on tree trunks, in which it 
grows most frequently, is of course decidedly acid, often 
reaching the highest degree here considered. Subaci 
reaction is shown by the occurrence of this species on 
schist rock near Widewater, Maryland. It has been re- 
ported on limestone at several localities, but at the one 
available for study, in the gorge above Natural Bridge, 
Virginia, which is probably typical, its roots are embed- 
ded in thick moss coating the limestone ledges. The 
upper part of this moss, in which most of the fern roots 
lie, is distinctly acid in reaction, and only the layers. 
nearest the rock have the acidity neutralized. As the 
spores of the fern must have fallen and started to grow 
in the acid portion of this moss, such occurrences are not 
an indication that it is other than an acid soil plant, at 
most somewhat tolerant of calcium. 
Woodsia glabella is well known to occur on the cliffs 
of calcareous gneiss at Willoughby Lake, Vermont, and 
repeated tests made there showed circumneutral reac- 
tions throughout, the fern avoiding strictly the patches 
of subacid soils which occur here and there in that 
region. This fern has been reported,-to be sure, in 
a few places where the rocks are not known to be calcar- 
eous, but most of its localities are definitely in limestone 
regions. The rarer Woodsia alpina (hyperborea) is found 
in the same localities but in more exposed situations, 
