222 AGEICULTURAL AND INDUSTEIAL BACTEEIOLOGY 



digesting cellulose slowly have been isolated, apparently the 

 most important types have not thus far been fcnind. 



Many species of bacteria have been isolated capable of 

 decomposing cellulose under aerobic conditions. Agar plate 

 cultures containing finely granulated precipitate of cellu- 

 lose, constitute a suitable substrate for the growth of such 

 organisms. The bacteria produce an enzyme, cytase, which 

 dissolves the cellulose in the area immediately about the 

 colony and this area then becomes clear. Certain species 

 belonging to the genus Actinomyces are also believed to be 

 of importance in the soil. They are very common upon 

 decaying plant roots, and are abundant in and on the 

 decomposing straw of manure piles. Certain molds, among 

 them many species of Penicillium, are known which actively 

 digest cellulose under aerobic conditions. 



The products resulting from' the action of microorganisms 

 upon cellulose have not been adequately investigated.' 

 Whether or not the cellulose is actually broken down into 

 sugar, for icxample, before it is utilized by the bacteria or 

 transformed further, is not known. It seems fairly certain, 

 however, that in some cases, at least, the intermediate prod- 

 ucts of the decomposition of the cellulose are utilized more 

 or less directly and sugar is not formed at all. 



Organisms capable of bringing about cellulose fermenta- 

 tion are of great importance in nature, particularly in the 

 soil. By the changes they bring about they make available 

 sources of energy to many other types of soil bacteria. 

 Their functions will be noted later in connection with soil 

 bacteriology. 



Fermentation of Pectin, the Process of Retting. — 

 Pectin is a gumlike compound closely related to the carbo- 

 hydrates, occurring generally in plant tissues. In con- 

 stitutes the major portion of the binding material which 

 goes to make up the structure known as the middle lamella. 

 A careful study of a cross-section of any plant tissue will 



