CHAPTER XXIV 



VEGETATION OF THE MARSH AND BOO 



In a marsh or bog the water-level rises to the surface of 

 the ground, or above it. The roots, rhizomes, and in 

 some cases the lower parts of the erect stems, are thus 

 situated in water, but the greater part of the assimilating 

 organs are in the air. This constitutes the chief differ- 

 ence between the marsh-plant and the true aquatic 

 (hydrophyte), in which the entire plant is in water. 

 There is, however, no sharp line of distinction between 

 the two, some plants growing equally well under either 

 condition — e.g., Polygonum amphibium and Apium inun- 

 datum (p. 28). The leaves of the marsh-plant standing 

 out into the air can transpire like those of an ordinary 

 land-plant, but the air is laden with moisture, and the 

 rate of transpiration is slow — i.e., the conditions favour 

 the development of a hygrophjiiic type of vegetation 

 (p. 63). Often, however, as in bogs, a considerable 

 quantity of humous acid is present, which hinders absorp- 

 tion, and in aU cases absorption practically ceases in 

 the winter, when the soil-water is very cold or possibly 

 frozen. To meet these adverse conditions, xerophytic 

 characters must be present in the plant. In the bog, 

 where absorption is difficult throughout the year, the 

 xeroph3rtic characters are often permanent ; but in the 

 sweeter marsh the plants usually exhibit xerophytic 

 characters {e.g., perennating structures) in the winter 

 only ; in the summer they are typical hygrophytes (see 

 hygrophilous tropophytes, pp. 62, 63). 



Annual plants are more abundant than in the aquatic 

 vegetation, but they are stiU few, and these are found 

 only in the drier parts, where the mean water-level is 

 never above the surface. The temperature of the soil 



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