ACTION OF PLANTS AND ANIMALS 181 



of the soil. Aughey lias found roots of tlie buffalo berry 

 {Shepherdia argophylla) penetrating tbe loess soils of Nebraska 

 to the depth of 50 feet. In the limestone caverns of the Southern 

 States, the writer has often been impressed by the number of long 

 thread-like rootlets, so fine as to be almost imperceptible, which 

 have found their way through rifts in the rocky roof. 



H. Carrington Bolton has shown that very many minerals 

 are decomposed by the action of cold citric acid for a more or 

 less prolonged period, the zeolites and other hydrous silicates 

 being especially susceptible- Such tests have great significance 

 when we consider that the roots of growing plants secrete an 

 acid sap, which, by actual experiment, has been found capable 

 of etching marble. The exact nature of this acid is not accurately 

 known, but it is considered probable that in the rootlets of each 

 species of plant there exists a considerable variety of organic 

 acids.^ 



But the effects of plant growth are not necessarily always 

 destructive; such may be conservative or even protective. In 

 glaciated regions, it is often the case that the striated and pol- 

 ished surfaces of the rocks have been preserved only where pro- 

 tected from the disintegrating action of the sun and atmosphere 

 by a thin layer of turf or moss. As a general rule, however, 

 the manifest action of plant growth is to accelerate chemical 

 decomposition, through keeping the surfaces continually moist, 

 and to retard erosion. 



Action of Bacteria. — The researches of A. Miintz,^ Wido- 

 gradsky, Schlosing, and others tend to show that bacteria may 

 exercise an important influence in promoting rock disintegra- 

 tion and decomposition. Their influence in promoting nitri- 

 fication has been already alluded to. It would appear that 

 while these organisms secrete and utilize for their sustenance 

 the carbon from the carbonic acid of the atmosphere, as do 

 plants of a higher order, they may also assimilate carbonate 

 of ammonium, forming from it organic matter and setting free 

 nitric acid. Being of microscopic proportions, the organisms 



^ See Application of Organic Acids to tte Examination of Minerals, H. 

 Carrington Bolton, Proc. Am. Assoc, for the Advancement of Science, XXXI, 

 1883, and Available Mineral Plant Pood in Soils, B. Dyer, Jour. Chem. 

 Society, March, 1894. Eeeent work seems to show that the corrosive effect 

 of root action as noted above may have been dne wholly to carbonic acid. 

 See Bull. 30, Bureau of Soils, U. S. Dept. of Agriculture. 



^Comptes Eendus de I'Acad^mie des Sciences, OX, 1890, p. 1370. 



