500 
On the Formation of Circular Muskeag in Tamarack Swamps. 
CONWAY MACMILLAN. 
(PLATES 279-281.) 
In 1892 observations made by the writer upon plant-distribu- 
tion in the valley of the Minnesota led him to recognize and de- 
scribe what were termed ¢ension-lnes between groups of plants 
established in their habitats. In addition to that principal tension 
between centrally and distally distributed floras arising from what 
was denoted as equatorial pressure, other tensions due to topo- 
graphic conditions were defined as secondary and as minor tensions. 
«By the latter term there is not meant the forest and prairie de- 
limitation, for that is to be referred in large part to the principal 
lateral tension, developed by equatorial pressure. The various 
topographical features of the Minnesota valley with its gorges, 
glens, vales, meadows, hills and headlands bring about slight but 
distinguishable segregations of floral elements. Between meadow 
and bluff there exists a minor tension-line; between swale and 
knoll on the prairie, between hill and ravine in the forest, there are 
to be discovered such minor tensions.”* In this paragraph was 
laid the foundation for a series of studies in the zonal distribution 
of plants, and much interesting material has since been reviewed. 
In an extended paper not yet published, but read before the Bo- 
tanical Society of America at the Buffalo meeting in 1896, and 
given in abstract in an American journal, + the zonal distribution 
of plants upon the soches moutoneés of a fresh-water archipelago 
and upon lake strand and sand-dune islands was discussed and 
certain intimate and remarkable connections between physio 
graphic and plant-distributional conditions were indicated. 
Zonal distribution was studied in 1893 by Ant. Magnin,{ who 
described the Carex, Phragmites, Scirpus, water-lily, pond-weed 
and Chara formations which, by their sequence, mark the increas~ 
ing depth of the water off shore in the lakes of the Jura. 4 | 
month or two later Eguisetum zonal distribution was noted in the 
—— 
* MacMillan, Metaspermae of the Minnesota Valley, 596, 1892. 
_ +The Botanical Gazette, 22: 218. 1896. 
$Magnin, A. Recherch. Veg. Lac. du Jura, Rey. Gen. Bot. 5: 241. 1893 
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