460 PRINCIPLES OF SOIL MICROBIOLOGY 



the organic complexes. 102 Among the higher fungi Merulius lacrimans 

 occupies the leading place. The course of decomposition is as follows: 

 The fungi first assimilate the sugars, then the dextrins and gums, later 

 the hemicelluloses, and finally the celluloses are acted upon. The physi- 

 cal properties are changed at the same time. The formation of so- 

 called "humus" in the soil, especially in forest soils, is in close connection 

 with the decomposition of wood ; the organic substance is first attacked 

 by fungi; a mixed flora of bacteria and fungi then follows, finally a rich 

 fauna of lower animals. Among the residual substances, which go to 

 form the so-called humus and humic acids, lignin occupies a leading 

 place. 



It may be added here that, as far as our present information is 

 concerned, corky and cutinized lamellae are not acted upon to any 

 extent by microorganisms. 104 



Pectins, mucilages and gums, and their decomposition by microorganisms. 

 A number of constituents of the vegetable organic matter are character- 

 ized by a colloidal slimy constituency; they are usually amorphous in 

 nature and soluble in water. Some are of a pentosan nature, like the 

 pectins; others are of a hexosan nature, like the levulosans. They 

 can thus be classified with the hemicelluloses. A number of sub- 

 stances synthesized by microorganisms belong here, namely the slime 

 of cellulose-decomposing bacteria, of nitrogen-fixing and other bacteria, 

 the mycodextrans and mycogalactans of fungi, etc. 



The intracellular substance (middle lamella) of parenchymatous 

 tissues of plants consists of pectic substances, or pectose, insoluble in 

 cold water. Heating for a few minutes in an acid solution transforms 

 the insoluble pectoses into soluble pectins; these change into pectic 

 acid on heating with alkali. 105 As pointed out above, pectins are readily 



102 Schellenberg, H. C. Die Holzersetzung als biologisches Problem. Vier- 

 teljahrschr. Naturf. Gesell. Zurich, 65: 30-31. 1920. (Centrbl. Bakt. II, 

 55: 351. 1922); Ward, H. M. Ann. Bot. 12: 565. 1898. 



103 Further information on the decomposition of wood and the organisms con- 

 cerned is given by Tubeuf, C. F. V. Holzzerstorende Pilze und Haltbarmachung 

 des Holzes. Lafar's Handb. techn. Mykol., 3: 286-333. 1904; Thaysen, A. C. 

 The action of bacteria on cellulose and lignified vegetable tissues. Fuel., 2: 

 274-276. 1923. (Chem. Abstr., 18: 700. 1924.) 



104 Miyoshi, M. Die Durchbohrung von Membranen durch Pilzfaden. Jahrb. 

 Wiss. Bot., 28: 269-289. 1895; Otto, 1916 (p. 265). 



io6 Devaux, H. Sur la nature de la lamelle moyenne dans les tissus moux. 

 Mem. Soc. Sci. phys. Nat. Bordeaux (6), 3: 89. 1903. (Bot. Centrbl., 96: 1. 

 1904) ; Compt. Rend. Acad. Sci., 162: 561-563. 1916; Conrad, C. M. A bio- 

 chemical study of the insoluble pectic substances in vegetables. Amer. Jour. 

 Bot. 13: 531-547. 1926. 



