110 Decomposition of Plant and Animal Residues 



Starch is readily decomposed by a large numlDer of microorgan- 

 isms. Among the fungi, certain species of Aspergillus, such as 



A. orijzae, and, among the bacteria, various spore-formers, such as 



B. amylovorus, B. incsenfcrictis, and B. macerans, are particularly 

 capable of attacking starch. The products of starch hydrolysis are 

 further broken down by microorganisms, through some of the reac- 

 tions shown above for the sugars. In addition to the highly special- 

 ized starch-decomposing organisms capable of producing powerful 

 diastatic or amylolytic enzymes, numerous other fungi, bacteria, and 

 actinomycetes are also capable of utilizing starch. 



Decomposition of Cellulose 



Cellulose, like starch, is a polymer of glucose. Because of its spe- 

 cific physical structure, however, and its resistance to most enzymes 

 and chemical reagents, it presents distinct problems as regards 

 decomposition in soils and in composts. The formation of specific 

 ceilulolytic enzymes can be demonstrated only with great difficulty. 

 Cellulose represents chemically a single type of compound. Be- 

 cause of difterences in the nature of the accompanying impurities, 

 celluloses of different origin may show distinctly different physical 

 properties. 



Cellulose predominates in fibrous and woody materials, such as 

 straw, stubble, weeds, grasses, leaves, branches, and twigs. In young 

 and succulent plants, the cell-wall material is proportionally low, 

 whereas the sugars, proteins, and soluble minerals are high. In 

 mature plants, the straw, stems, leaves, and twigs are high in cellu- 

 lose. Cellulose is resistant to various oxidizing agents and is hy- 

 drolyzed only by concentrated acids. It is also resistant to attack 

 by the great majority of soil-inhabiting microorganisms. It can be 

 decomposed readily, however, by certain specific organisms found 

 among the bacteria, fungi, actinomycetes, and lower animals. 



Various systems have been proposed for classifying the cellulose- 

 decomposing organisms. They can be divided into a number of 

 distinct groups, on the basis of either morphological or physiological 

 differences. One such system comprises (1) aerobic bacteria, (2) 

 myxobacteria, (3) anaerobic bacteria, including thermophilic forms, 

 (4) actinomycetes, (5) filamentous fungi, (6) higher or mushroom 

 fungi, (7) protozoa, (S) insects and other animal forms. 



The mechanism of the breakdown of cellulose by microorganisms 

 depends entirely upon the nature of the organism and the condi- 



