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Harvey Stallard. 
are not attained in any of the bogs studied. Variations in the size, 
shape, depth, and situation of the atolls indicate that their forma¬ 
tion, is independent of such factors. 
The primary development of a bog is not as one would infer 
from the hypothesis set forth by MacMillan, and before entering 
upon a discussion of the effects of shade and toxic substances it 
will be necessary to state briefly the development of the bog. This 
development has been sketched in the “ Development of Climax 
Formations of Northern Minnesota” (4), and it seems sufficient to 
mention here those stages of the primary hydrarch succession of 
which the bog is a part. The following associes are usually 
representative stages of the bog’s development:— 
1. Chara-Philotria Associes. 
2. Castalia-Nymphcea Associes. 
3. Scirpus-Zizania Associes. 
4. Carex Associes. 
5. Andromeda-Chamcedaphne Associes. 
6. Larix-Picea Associes. 
Sphagnum enters with or after Carex and along with Andro¬ 
meda and Chamcedaphne. Just how much the development of the 
bog depends upon the Sphagnum which grows in the water pre- 
ceeding Carex, has not been determined. In most cases it is the 
Sphagnum which follows the Carex that plays the most important 
role in filling up the pond. 
The succession begins naturally with the shore of the pond, 
and its centripetal advance if not disturbed presents a zonation 
corresponding to the stages of succession as long as there is a 
remnant of the pond. In undisturbed areas there is no zone of 
water near the margin of the parent pond. The advance of the 
shade-casting trees is never so rapid that it halts the succession 
by killing out Sphagnum. The latter must develop to prepare the 
way for the trees, and the growth of the tamarack and spruce is so 
slow that several inches of peat may be deposited before there is 
sufficient shade to destroy the Sphagnum. The toxic substance 
supposed to interfere with the development of the outer part of the 
bog is calcium carbonate. The presence of calcium carbonate 
however does not seem to interfere with the normal sequence of 
the associes given below. Lakes and bogs containing large deposits 
of calcareous marl present the same prisere as those devoid of 
lime. Sphagnum, which lime salts affect, enters the succession 
after the Scirpus-Zizania and the Carex associes have deposited a 
