284 MINNESOTA BOTANICAL STUDIES 



All of these species are densely social but this tendency is 

 developed to the most exclusive extreme in Phragmites. The closely 

 aggregated stems with copious leafage in this plant result in the 

 establishment of a control that is often absolute. Within such dense 

 stands the sunlight seldom penetrates to the water level and hence 

 the presence of additional species is rarely noted in such places. 

 Vigorous creeping stems anchor the plants securely, and, although 

 these stands are sometimes exposed to powerful winds, the soil 

 connection of the dense communities is never broken. 



The bulrush ranks little below the reed-grass in density. As a 

 matter of fact the number of erect stems may be greater in the 

 former, but, because of the leafless nature of these stems, even the 

 densest aggregation of bulrushes admit considerably more light than 

 enters the reed-grass stands. The bulrush is more often seen in 

 open association than is either of the other dominants. 



The cat-tail occurs in close stands also but even with a heavy 

 leafage the disposition of the leaves is such that the species never 

 forms as exclusive stands as does Phragmites. 



Wild rice, an annual non-rhizomatous species, is also seen in 

 rather close stands, but because of its annual life cycle does not play 

 the significant role in this formation peculiar to Scirpus, Phragmites, 

 and Typha. 



Throughout the region under study the bulrush exercises its 

 common role as a pioneer in the invasion of water areas. This 

 species usually encroaches upon the open water of the pond or lake 

 by sending out scattered rhizomes which fasten themselves in the 

 mass of debris laid down by the water-plant formation. From 

 these first few elongating rhizomes there arises a belt of helophilous 

 vegetation, characterized by widely scattered erect stems or more or 

 less isolated patches of scattered individuals. Tansley has sug- 

 gested (66) that "it is probable that this species can germinate from 

 submerged seed at a considerable distance from the land." This 

 may help explain the presence of scattered individuals or clumps 

 far beyond the well-defined marginal belt. Beds of muck, occupied 

 more or less scantily by submerged water-weeds, would certainly 

 afford a prime seed bed for such species. From these outlying cen- 

 ters of distribution additional rhizomes are sent forth that result in 

 the gradual increase of bottom areas held by the species and the 

 consequent extension of the emerged portion of the association. 



Wild rice invades in a similar manner although this species is 



