450 George E. Nichols, 



cushion- forming sphagnums is their great ability to absorb and 

 retain liquids. But while this in itself is a factor of no little 

 significance in hindering the loss of water, even more significant 

 is the manner in which individual clumps of these mosses run 

 together and form banks which may obstruct the drainage to 

 such an extent that in favorable situations, as on gentle slopes, 

 the water may be dammed back to form ponds and pools of 

 various dimensions. The degree to which masses of sphagnums 

 are thus able to hold back the water is remarkable. In the boggy 

 area diagrammatically shown by Fig. 65, C, for example, the 

 level of the water in the pond at e is nine inches higher than that 

 at d, twenty-five feet distant; and the water level in pond 

 d is twelve inches above that in pond c, equally distant. In 

 another instance a difference in elevation of two feet was 

 measured between two water surfaces thirty-five feet apart; 

 while in two other cases differences in level amounting respectively 

 to nearly ten feet in less than a hundred, and to more than one 

 foot in three were estimated. On the "down-hill" sides of a pond 

 the banks of sphagnum rise steeply from the water's edge to a 

 height of one, two, or more feet above the pond's surface. In 

 one instance a rise of three feet within seven feet of the water's 

 edge (or to a height of about five feet above the mucky bottom 

 of the pond) was noted. It is obvious that these ponds, by 

 retaining much of the water which accumulates in them during 

 wet periods, or which drains into them from higher levels, 

 function as storage reservoirs and insure to adjoining areas a 

 fairly uniform water supply throughout the season. 



Incipient ponds of the sort just described are frequently 

 encountered in the bog meadow stage of the succession, but there 

 they are usually shallow and ephemeral. It is in the wet bog 

 stage that they first attain a position of ecological importance. 

 The formation of ponds hastens the elimination of the bog 

 meadow as a distinct association-type, for their spread leads 

 naturally to the extermination of any plants which may have 

 tenanted the areas which they now occupy, except for the few 

 species which are able to adapt themselves to the changed condi- 

 tions, either by assuming an aquatic habit (e. g., Sphagnum 

 Pylaisei) or through their position above the water level (e. g., 

 tussocks of cushion-forming sphagnums). 



