a 
1913] COOPER—ISLE ROYALE 205 
edge of the sphagnum, and occurs to a limited extent some distance 
back in the moss. Chamaedaphne is also found commonly upon 
the sedge mat, but inhabits the drier portions. In the sphagnum 
area it is abundant and over a wide belt almost the only shrub, 
extending back until it meets the zone where Ledum groenlandicum 
is the dominant species. Relative ability to withstand extreme wet 
bog soil conditions determines this zonation at the beginning, but 
the sudden elimination of Andromeda, leaving Chamaedaphne in full 
control, is due principally to the smothering effect of the sphagnum, 
which the former shrub is unable to avoid. Chamaedaphne, on the 
other hand, is able to grow up with the moss indefinitely, and there- 
fore persists until the entrance of Ledwm introduces a new factor. 
It is not certain that Chamaedaphne does not sometimes germinate 
upon the surface of the sphagnum as well as upon the sedge mat, 
and thus in part maintain its dominance. It is certain, however, 
that Andromeda does not commonly do so, at least not successfully. 
Two other shrubs, Kalmia polifolia Wang (pale laurel) and Betula 
pumila L. (dwarf birch), occur in this and other bogs, but not in 
sufficient abundance for satisfactory study of their habits. 
Upon the surface of the sphagnum another group of species 
becomes established. Important among these are Carex pauciflora 
Lightf., Smilacina trifolia (L.) Desf. (three-leaved Solomon’s seal), 
Drosera rotundifolia L. (round-leaved sundew), Vaccinium Oxycoc- 
cos L. (small cranberry). All of these are able in greater or less 
degree to keep pace with’ the continued upward growth of the moss. 
Of far greater importance than these is Ledum groenlandicum 
Oeder (Labrador tea), which becomes established long after the 
other shrubs, indeed after all but Chamaedaphne, have disappeared. 
Ledum is almost invariably found to be related definitely to the 
sphagnum, its whole root system being contained within the mass. 
The growth that it forms is very dense (fig. 42), and as it is a 
taller shrub than Chamaedaphne it shades it severely, and thus 
finally causes its elimination. Its effect upon the sphagnum is 
similar. Because of the shade which Ledum produces and the 
considerable amount of waste which falls from it, the upward 
growth of the moss is gradually retarded and finally ceases 
altogether. 
