DEVELOPMENT OF PLANTS 



269 



are popularly referred to as sour, and as a result, you will always 

 find associated with the bog mosses a rather limited and peculiar 

 variety of plants such as several genera of heaths, sedges, or- 

 chids, pitcher plants (Sarracenia) and other insectivorous plants 

 like the sundew (Drosera). For some reason the ordinary 

 plant is not able to procure its food frorii these sour bogs and 

 this may explain the common occurrence in such places of in- 

 sectivorous plants, and of certain trees and shrubs that are asso- 

 ciated with mycorrhiza. The sphagnums grow luxuriantly in 

 such places, the lower portions of the stem 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 



FIG. 20 1. 



FIG. 202. 



FIG. 201. The bog mass, 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 openings. 



