DEVELOPMENT OF PLANTS 



297 



dying off and the upper portion branching and continuing the 

 growth from year to year. In this way, ponds are gradually 

 covered with a layer of mosses which become rather insecurely 

 bound together by the subsequent introduction of other plants 

 whose roots or stems traverse the covering formed by the mosses. 

 So frail is this covering at first, that a very slight jar will cause 

 the surface to tremble and for this reason these places are known 



FIG. 201. 



FIG. 202. 



FIG. 201. The bog moss, Sphagnum, bearing three sporophytes and 

 numerous lateral branches covered with closely overlapping leaves. Note 

 that some of these branches envelop the stem in wick-like strands. 



FIG. 202. Structure of the leaf of Sphagnum: A, section of leaf, showing 

 its single layer of cells that consists of large empty cells alternating with 

 small chlorophyll-bearing ones. B t surface view of the cells, the larger empty 

 cells being marked with spiral bands and often perforated with minute open- 

 ing. 



as quaking bogs. The constant dropping of the decaying vege- 

 tation to the bottom of the ponds, assisted by the drainage 

 material, gradually fills them, and in time they may become 

 quite dry. The acid character of these bogs prevents the entrance 

 of those organisms which promote decay. As a result, plants 

 20 



