424 George E. NicJioIs, 



accomplished entirely through one or the other. But both pro- 

 cesses may commonly be observed in one and the same pond 

 and, as will be shown later, the filling in and obliteration of the 

 pond may frequently be achieved through the combination of 

 the two. 



Filling from within, with particular reference to the role of 

 various sphagnums. — The commonly active participation in this 

 process of various aquatic seed plants and algae calls for no 

 special comment. Of more interest here is the conspicuous part 

 often taken by certain species of sphagnum. These are particu- 

 larly important in the barrens where, in the small, undrained 

 ponds which are so abundantly developed and which constitute 

 such a characteristic feature of the plateau, various sphagnums 

 commonly predominate the successive stages of bog evolution 

 from start to finish. It is to the conditions observed in and 

 about these ponds that the following remarks apply. Here, 

 while a subordinate part may frequently be played by various 

 other plants, the bulk of the organic debris by which the pond 

 becomes filled in up to water level is contributed by the two 

 aquatic sphagnums, 5". Pylaisei and 6^. ciispidatum, associated 

 with which, but in lesser abundance, usually grows the liver- 

 wort Cephalosia fluitans. During the summer months these two 

 species of Sphagnum, either or both, may be present in such pro- 

 fusion as to completely clog the pond to a depth of several inches 

 below the surface with a loose, floating mass of vegetation. One 

 is tempted to regard such a structure as a true floating mat, 

 but such is hardly the case (see, however, in this connection, 

 p. 429). For while the "mat" does float during the growing 

 season, so long as there is open water underneath it sinks to the 

 bottom in winter. It is indeed extremely doubtful whether 

 under any circumstances sphagnum of itself is capable of form- 

 ing a permanent floating mat, i. e., a raft-like growth sufficiently 

 firm and stable to permit the establishment and maintenance on 

 its surface of a non-aquatic type of vegetation. 



Eventually, however, the bottom of the pond may become built 

 up to such a level that, except during periods of high water, 

 the substratum is exposed to the air, and here, in addition to the 

 bryophytes which have been largely responsible for its develop- 

 ment and which still cover its surface with a thin, more or less 

 continuous mat, the soft, mucky ground usually becomes popu- 



