Hydrogen Ion Concentration 269 



and fungi tolerate values as low as pH 1.4. Other species of animals 

 and plants live in highly alkaline waters: Lake Elmenteita in the 

 African Rift with a pH of 10.7 to 11.2 supports a considerable fauna 

 and flora (Jenkin, 1936), and Soap Lake, Washington, with a />H 

 running up to 10 is found to contain a tremendous population of 

 rotifers and cladocerans during the summer. In contrast to these 

 examples of wide toleration, are instances of organisms that will live 

 only within a very narrow range of hydrogen ion concentration, 

 sometimes extending over less than a pH unit. The ciliated pro- 

 tozoan Stentor coeruleus is reported limited to pH 7.7 to 8.0. These 

 aspects of pH and other physicochemical factors of the aquatic en- 

 vironment are reviewed more extensively by Allee et al. (1949, Ch. 

 11), and by Welch (1952, Ch. 7). 



The pH of mud and of soils is frequently one of the most important 

 characteristics of these substrata. A good general account of the 

 ecological aspects of this highly complex subject will be found in 

 Daubenmire (1947, Ch, 2), and more detailed treatments are avail- 

 able in ZoBell (1946) and Waksman (1932). As we have seen, the 

 pH of these aquatic and terrestrial substrata varies widely, depending 

 on the nature of the parent material, the degree of weathering, and 

 the extent of biological activity, including decomposition. Organisms 

 inhabiting mud must be capable of withstanding pH values that may 

 be considerably different from those of the overlying water. The 

 floras of strongly acid soils, such as those developed from granitic 

 rocks in the cold temperate regions, are characteristically different 

 from the floras of alkaline limestone soils. Fungi are the chief or- 

 ganisms of decay in acid forest soils, whereas acid-sensitive bacteria 

 and earthworms are abundant only in soils with a more nearly neutral 

 reaction. The striking difference even in the optimum pH range of 

 plants is illustrated by the accompanying examples (Spurway, 1941). 



Bog rosemary {Andromeda glaucophylla) 3.0-5.0 



Blueberry {Vaccinium cnrymbnsum) 4.0-5.0 



Wake robin {Trillium erectuin) 7.0-7.5 



Spleenwort {Asplenium parvulvm) 7.0-8.5 



Stinking cliara {Chara vulgaris) 7.5-8.5 



Plants frequently modify the pH of their substratum by the action of 

 their root secretions and their decomposition products. This activity 

 accounts for the occasional occurrence in limestone regions of species 

 characteristic of acid soils. In such instances the pioneer plants have 

 probably established themselves in microhabitats of lower pH, and 



