DEVELOPMENT OF PLANTS 



283 



traverse the covering formed by the mosses. So frail is this 

 covering at first, that a very sHght jar will cause the surface to 

 tremble and for this reason these places are known as quaking 

 bogs. The constant dropping of the decaying vegetation 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 and animals that fall 



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, surface view of the cells, the larger empty 

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

 ings. 



into these bogs may only partly decay and remain for ages in a 

 wonderful state of preservation as is shown by the recovery of 

 skeletons of animals and clothing of men that belonged to a 



