204 
GOTE TURESSON 
stitution bog and replaced by Philonotis fontana. This was particu- 
larly noticeable in the vicinity of Ledum thickets where the shade 
was dense and waste abundant. An observation somewhat similar 
to that of Eriksson can be made in the Green Lake bog. Slight de- 
pressions are found beneath the hemlocks, the Sphagnum being 
unable to grow here on account of the shade. The trees are not killed, 
however, as Tsuga is able to endure very wet soils. 
The origin of the marginal ditch surrounding sphagnum bogs in 
some regions has been discussed from time to time. In order to reach 
a satisfactory explanation we have to consider not only the factor of 
shade as expounded by Cooper (4) and Atkinson (i), but also the 
factor of waste. Fallen leaves and other organic material are swept 
from the forest floor into the edge of the pool and tend to smother the 
vegetation (Shaw 23). The combined factor of shade and waste is 
very likely responsible for the marginal ditch, although suggestions 
of another nature are made by some. (Burns 3, Grabner in Warming 
28). 
It has been shown how Lysichiton is able to hold its own in the 
sphagnum bog and how as a consequence deep pits are formed. In 
order to determine more definitely the relation of Sphagnum to 
Lysichiton and if possible to ascertain in which stage and at what 
time in the history of the peat bog Lysichiton made its entrance, a 
cross-section was made through one of the skunk cabbage pits ex- 
tending one meter beyond the pit and two meters down through the 
peat. The analyses of the samples of peat brought up from this 
ditch were supplemented with those made from a ditch already dug 
in the peat bog for the purpose of draining the bog. To complete 
the data on the stratification of the bog, test borings were made from 
the bottom of these ditches. Figure 5 shows the nature of the strati- 
fication of this peat deposit. 
The area now occupied by the peat deposits is evidently an old 
lake basin on a bed of drift clay. The outlet of the former lake was 
obstructed relatively early and the course of development followed 
that of the undrained swamp. This is confirmed by the entire ab- 
sence of remains of alder, deciduous shrubs and other plants of the 
half-drained swamp. That the development has gone through the 
bog sedge meadow, poorly represented on the mainland at present, 
is seen from the thick layer of Carex peat overlying the clay. From 
a wet stage the bog reached a drier stage and big trees grew up. The 
