250 
Harvey Stallard. 
(c) The sporophyte arises as an intercalated stage in the life- 
history, due to the elaboration of the zygote ( antithetic alternation ):— 
(1) The gametophyte arises from the prostrate system 
(Coleochcete, Hepaticae). 
(2) The gametophyte arises from the erect system 
(Nemalionales). 
(3) The gametophyte retains both the prostrate and erect 
systems ( Batrachospermum , Musci). 
(d) There are two spore-producing generations, the one an 
intercalated phase produced from the zygote, the other strictly 
homologous ( Polysiplionia , Griffithsia and other advanced Florideae). 
East London College, 
November 28th, 1916. 
THE ORIGIN OF SPHAGNUM ATOLLS. 
By Harvey Stallard. 
(University of Minnesota). 
I N connection with the study of the development of the climax 
vegetation of Minnesota, opportunity has been afforded the 
writer to investigate the sphagnum atolls of the state and par¬ 
ticularly the Anderson and Ballard atolls of Crow Wing County 
which MacMillan (1) described in 1894. A sphagnum atoll is any 
bog surrounded by a trench of stagnant water. In the centre of 
the bog the remains of the original pond may be present in which 
case the name atoll is quite applicable. Such bogs are to be found 
in northern and central Minnesota and are said to occur in 
Wisconsin, Michigan, and New York. 
MacMillan (1) endeavoured to explain their development and 
suggested probable causes of their formation. He attributed their 
development primarily to the rise and fall of the water level of the 
original pond, and summarises the process as follows: “The 
origin of the sphagnum atolls in the cases studied may be ascribed 
to a season of gradual recession of the waters of the pond, followed 
by a season of comparatively rapid increase in area and level. The 
atolls first appear as annular floating bogs separated from the shore¬ 
ward turf as a result of the original zonal distribution of littoral 
plants and the rise of the waters together with the favourable 
concurrence of a group of special and necessary conditions. Some 
of the apparent conditions of the atoll-formation are (a) a definite 
maximum size and depth of the parent pond; (b) considerable 
