CHAPTER XXIV 
VEGETATION OF THE MARSH AND BOG 
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—.e., the conditions favour 
the development of a hygrophytic type of vegetation 
(p. 63). Often, however, as in bogs, a considerable 
quantity of humous acid is present, which hinders absorp- 
tion, and in all 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 
xerophytic 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 still 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 
