230 BOTANICAL GAZETTE [MARCH 
Two lines of succession are distinguished after the sedge mat 
stage. One is characterized by Chamaedaphne, Andromeda, and 
Alnus incana in the shrub stage, practical absence of sphagnum, and 
by Larix and sometimes Thuja in the bog forest; the other by 
Chamaedaphne and Andromeda followed by Ledum in the shrub 
stage, abundance of sphagnum accompanying the shrubs, and by 
Larix and Picea mariana in.the bog forest. Sphagnum is the critical 
plant in the differentiation of the two series, since Ledum and Picea 
mariana appear later, and only in cases where sphagnum is 
abundant. The differences may be related to differences in drain- 
age, since those bogs containing little sphagnum are usually well 
drained, while in those with abundance of sphagnum, as far as 
observation has gone, drainage was very poor or lacking entirely. 
A contributing factor is found in differences in the composition of 
the sedge mat preceding the shrubs and sphagnum. In the sphag- 
num bogs Carex limosa is the principal mat-forming species. Being 
low and soft, it offers no resistance to the spread of the moss. In 
the bogs with little sphagnum Carex filiformis is the important 
mat-former. On account of its height and stiffness and dense 
growth it produces unfavorable conditions for the spread of sphag- 
num. The reason for the differing distribution of the two carices 
is unknown; it may be merely accidental. 
The sphagnum is a superficial layer supported upon the sedge 
mat, and contributes little toward peat formation. It begins 
growth some distance within the bog margin and spreads both ways, 
slowly toward the margin, faster centerward. The area between 
the sphagnum and the upland commonly forms a marginal trench. 
In some places the moss by recent invasion has obliterated the mar- 
ginal trench, and occasionally it transgresses the bog margin, 
spreading up the forest floor for several meters. 
The sphagnum spreads marginally, surrounding and smother- 
ing such plants as cannot keep pace with its growth. Certain 
species by upward elongation are able to survive for some time, 
especially Andromeda and Chamaedaphne, the latter persisting 
longest. 
Ledum almost invariably follows the sphagnum, and its root 
system is usually strictly confined to the masses of it. It forms 4 
