Boreal Associations and Forests. 207 
groenlandicum), Rubus hispidus, Comarum palustre, Andromeda polifolia, Chio- 
genes hispidula, Vaccinium Oxycoccus, Eriophorum virginicum, Rhododendron 
Rhodora, was gradually transformed into firmer ground. Such shrubs as Cas- 
sandra (Chamaedaphne) calyculata, Vaccinium corymbosum, Asalea viscosa, Ilex 
verticillata, Rosa carolina, Pyrus arbutifolia, Nemopanthes fascicularis and 
young trees, Acer rubrum, Betula populifolia, Betula pumila, Picea nıgra 
(= P. mariana) and Larir americana (= L. laricina) began to smother out 
the typic bog plants. The area thus encroached upon culminated in a climax 
forest association. Such a sequence of events one can trace in the coniferous 
areas of Wisconsin, Minnesota, New York, northern Pennsylvania, the New 
England states and Canada‘). As an evidence of the constant shifting of floral 
types in a region may be mentioned the conversion of a former sphagnum bog 
area into a salt marsh. PENHALLOW ’°) finds upon examination of the strata in 
a number of salt marshes on the New England coast that their history is as 
follows: first-sphagnum bogs existed and were subsequently invaded by sedges, 
Cassandra (Chamaedaphne) calyculata and finally the white pine, Pinus strobus. 
Suddenly by the disappearance of the sea barrier, the former bog areas were 
flooded with salt water and converted into, salt marshes. PENHALLOW cal- 
culates from the character of the plant remains that an indefinite period 
may be assigned to the formation of the bogs, but that the Cassandra-layer 
(4—5 inches) was formed in 50 years, the second peat layer with small white 
pine stumps in 200 years, the first peat zone with larger white pine trees 
in zo years and the superimposed salt marshes 100 years to form, a perio 
of 420 years after the bogs had been captured by vegetation other Ban sphag- 
num mosses. 
The swamp plant associations of North America next concern us. As 
we know from numerous physiographic studies that have been made of glacial 
basins, many of the lakes were formerly much larger than at present. Some 
of them in early post glacial times had steep banks, which were unfavorable 
to the development of shore vegetation. But by the lowering of the water 
level consequent upon the cutting down of the outlet, the shore line at present 
‘ is a gradually sloping one and supports a “drained swamp” flora. In other 
cases, irregular arms extending away from the main body of the lake and 
protected from wind and wave action, doubtless supported a bog vegetation 
during the tundra dominance. Since then they have been separated from the 
main lake by a lowering of the water level. To day we find in such cases 
the bog vegetation still persisting in the depressions which were formerly arms 
of the lake, while on the shore of the main body of water, which came to be 
swampy at a much later period, the so-called “drained swamp” flora occurs. 
The relation of these two distinct associations of plants seems to depend largely 
1) This account is based upon the observations of the writer upon kettlehole bogs in the 
great terminal moraine of the Pocono plateau of northeastern Pennsylvania and of E. N. TRANSEAU 
l.c. in Michigan and North America generally. 
2) Transact. of the R. Soc. of Canada ser. 3, I sect. 4: 13—45- 
