VEGETATION OF ROCKY PLACES. 615 



likelihood that the pond will cease to exist, yielding, as it has 

 in other places, the right of possession to the encroaching vege- 

 tation. 



How was the atoll formed? In the early formation of the 

 atoll it is possible that certain of the water-loving carices and 

 grasses began to grow some distance (three or four meters) 

 from the shore, where the water was of a depth suited to their 

 habit. The stools of these plants gradually came nearer the 

 surface of the water. As they approach the surface other plants, 

 not so strong-rooted, like mosses, sphagnum, etc., find anchorage, 

 and are also protected to some extent from the direct rays of 

 sunlight. Partial disintegration of the dead plant parts, which, 

 mingling with the soil, gradually fills on the inside of the zone, 

 so that the depth of the water there becomes less. Now the 

 zone of the carices can be extended inward. 



The continued growth of the sphagnum and the dying away 

 of the lower part of the plant add to the bulk of the plant-remains 

 in the zone, and finally quite a firm ground is formed, shutting 

 off the shallow water near the shore from the deeper water of the 

 pond. As time goes on other plants enter and complicate the 

 formation, and even make new ones, as when the cassandra 

 takes possession. 



The original pond here was rather oblong, and one end possi- 

 bly much shallower than the other, so that it filled in much more 

 rapidly, leaving the central pond at the east end. Over a por- 

 tion of the west end there is an extensive cassandra formation, 

 with some ledum (Labrador tea) , but separated from the circular 

 cassandra zone by an intermediate zone. In this end-cassandra 

 formation other shrubs, and white pines five to fifteen years old, 

 are gaining a foothold, and in a quarter of a century or more, 

 if left undisturbed, one may expect considerable changes in the 

 flora of this atoll. 



The ditch on the margin of the atoll-moor was probably formed 

 because of the shade offered by the forest on the banks. On 

 the side where the original forest still stands the ditch is deeper, 

 and where it is heavily shaded no vegetation is present close to 



