rort] DACHNOWSKI—CRANBERRY ISLAND 25 
and various secondary dependent associates, occupy the firmer 
parts of the marginal zone and form an almost continuous fringe, 
the Alnus-Rhus association. In places it extends diagonally 
across the bog island as scattered dense thickets (fig. 4). This 
community of plants presents on the whole very little zonation 
within itself. It constitutes a zone of varying width, 5-30 feet 
(t.5-9 m.) and more, and attains a height of 8-12 feet (2.5-3.5 m.). 
Only in a few places along the southern shore this type of bog 
shrub formation is absent altogether and is replaced, as has been 
Fic. 4.—Map of Cranberry Island; surveyed February 1910; the divisions into 
plant societies as indicated by the map and the text are based on general characters 
of the vegetation; A, Decodon; T, Typha; the ponds on the island are shaded. 
stated, by Decodon and Typha. The edaphic conditions of this 
part of the habitat seem to approximate those of the undrained 
swamps as described by Cowtes (6). Nearer the lake there is a 
tendency toward the segregation of Decodon verticillatus and 
Hibiscus Moscheutos. Of the two, the former is more vigorous 
and occupies the deeper water. Rosa carolina prefers the outer 
border also, but clings quite closely to Alnus incana and Cornus 
stolonifera. Contemporaneous with the thicket-formers, various 
species of lianas invade the association. The mature thickets are 
often covered with an impenetrable growth of tangled vines of 
Apios tuberosa, Solanum Dulcamara, Convoloulus Sepium, Ipomoea 
sp., and Cuscuta Gronovii. Cephalanthus occidentalis does not 
