202 BOTANICAL GAZETTE [MARCH 
to be invading the latter area. In the opposite direction (center- 
ward) the sphagnum decreases gradually in thickness, and at its 
edge invasion of the sedge zone is actively taking place. Fig. 39, 
drawn to scale, is a typical bog section. The form of the ridge is 
shown and also the depth at various points. The high projection 
upon the sphagnum mass is a hummock, the true marginal face 
being at the right. A tongue of the moss is seen invading the 
marginal zone. Below the line which is drawn as marking the 
base of the sphagnum the soil is black peat containing little that 
is recognizable even with a microscope. At several places, however, 
sphagnum fragments were recognized in various degrees of abun- 
dance some centimeters below the line indicated. These were 
a b c d e 
LLL» Me — eB Dore Laie ho 7 
Peat d 
Bed Rock 
TWN ~ 
oes es 
Fic. 39.—Section through fiokoky Island bog: a, Sphagnum-Chamaedaphne; 
b, Sphagnum-Ledum; c, Sphagnum invading marginal trench; d, marginal trench; 
¢, upland; 1, sedge zone; 2, Calliergon Schreberi; 3, Calliergon Schreberi (fossils) ; 
4, Carex trisperma and Hylocomium proliferum; 5, Hylocomium proliferum; 6, 7; 
Drepanocladus vernicosus (?) (fossils). 
probably washed down from above, since this line certainly indi- 
cates the plane at which the growth began. 
The development of the sphagnum has not made uninterrupted 
progress, for at 3 on the section a stratum was found which con- 
tained abundant fragments of Calliergon Schreberi, and at 6 and 
7 remains of Drepanocladus vernicosus (Lindb.) Warnst. were 
discovered. These two species evidently obtained a foothold 
upon the surface of the sphagnum and for a time arrested its growth 
over certain areas. The latter again gained the upper hand and 
buried the invaders. 
The zone included between the sphagnum and the slopes of the 
ee commonly takes the form of a circular trench. This 
‘marginal trench” is a widespread feature of bogs, and various 
