LYSICHITON CAMTSCHATCENSE (l.) SCHOTT 
197 
also in Spanaway Lake, near Tacoma; but neither is typical of the 
bog so well represented on Mount Constitution in the San Juan Islands. 
It might be a matter of difference in age between the bogs on the 
mainland and those of San Juan Islands, the former having advanced 
further than the latter and obliterated the sedge bog stage, but there 
are grounds for the belief that climatic factors are responsible for the 
dissimilarity. 
The San Juan Islands, situated between the Strait of Juan de 
Fuca and the Strait of Georgia in the northern part of Puget Sound, 
have a lesser rainfall than most places on the mainland, probably 
because of the fact that they lie in lee of the Olympic Mountains 
(Piper, 19). The result is a difference in the flora of the two regions 
which in some respects is quite remarkable (Piper, 19, Turesson 27). 
Many plants characteristic of the drier eastern parts of Washington 
occur on the islands while they are absent from the intermediate 
more humid regions. The similarity between the swamp flora of the 
San Juan Islands and the arid parts of eastern Washington is also 
great. Indeed, the composition of the swamp vegetation around 
Sportsman Lake on San Juan Island is almost identical with that of 
Newman Lake and Liberty Lake of Spokane County in eastern Wash- 
ington. Both regions are too arid for Sphagnum. It is true, Sphag- 
num occurs in the northeast corner of Sportsman Lake, but it is not 
peat forming and cannot exist outside of the protective zone of willow 
thickets. The southern end of the lake has an extensive association 
of Ledum groenlandicum but Sphagnum is absent. Similarly, the 
sedge bog meadow on Mount Constitution has great resemblance to 
the sedge bog meadows in eastern Washington and it is only the 
slightly higher altitude, and .as a result the somewhat greater humidity 
that makes peat formation through Sphagnum possible on the former 
locality. However, on the mainland where a more humid climate 
favors the growth of Sphagnum, the development of the bog becomes 
altered due to the early ingress of the white moss. The edaphic 
conditions for a luxuriant growth of Sphagnum are fullfilled already 
before the establishing of the floating mat vegetation. The inter- 
polation of the floating mat stage of a purely hydrophilous vegetation 
cannot in itself represent a step towards more xerophytic conditions 
favoring the growth of Sphagnum and the sphagnophilous flora. 
These are, as stated above, already in evidence before the floating 
mat stage has been reached, and the mat merely serves as mechanical 
