No. 6. DEPARTMENT OF AGRICULTURE. 323 



rue entire bulk of plant structures, are included under the heads 

 (1) cellulose, (2) proteid matter, (3) carbohydrates and (4) fats and oils. 

 The decomposition of these several materials in the soil will be con- 

 sidered each in turn. 



3. The Decomposition of Cellulose or Vegetable Fibre. 



That straw and bits of woody fibre become soft and finally disap- 

 pear as such when incorporated with the soil is a fact of common 

 observation. Leaves and stems when mingled with earth rapidly 

 lose their structural characteristics and become converted into a 

 shapeless mass of mould. The log or stump under the action of bio- 

 logical and chemical agents decays more slowly, but eventually looses 

 its structure and becomes converted into a brownish pulverent de- 

 bris. 



These changes involve the fermentation of cellulose or vegetable 

 fibre, and are of special interest. The walls of vegetable cells are 

 composed of matter more or less complex in character; but since 

 cellulose in one form or another constitutes the basal portion of all 

 cell walls, it has been common to refer to them as composed of this 

 substance. But more accurately speaking cellulose is now under- 

 stood to include a large class of plant constituents. 



These latter may be grouped under two heads: (1) the celluloses 

 and (2) the pectoses. The walls of different cells differ in the rela- 

 tive proportion of these two classes of bodies. Thus the walls of 

 cells which constitute so called succulent or parenchyma tissue are 

 relatively rich in pectoses. This is particularly marked in the flesh 

 of fruits. Cellulose differs in its properties and ability to undergo 

 fermentative changes. In the latter respect cotton fibres are the 

 pjost resistant and the cellulose of seeds the least so, while that 

 found in the fundamental tissue of the higher plants occupies an in- 

 termediate position. With the difference in the constitution of the 

 cell walls of plants there results a marked divergence in their ability 

 to undergo fermentative changes, and also a difference in the pro- 

 ducts of such fermentation. For this reason the fermentative de- 

 composition of cellulose becomes an extremely complex phenomenon. 



The dissolution of cellulose is brought about by the action of a fer- 

 ment or enzyme known as cytase. 



In 1886, DeBary,^ found in the fungus Peziza sclerotiorum. a sub- 

 stance which possesses the property of causing cell walls to swell, 

 become gelatinous, and in a measure to dissolve. Two years later 

 H .Marshall Ward,^" found that a similar ferment was secreted bv s 

 species of Botrytis, commonly associated with the soft rot of a num- 

 ber of cultivated plants. 



In his study of the latter, the author observed minute drops ex- 



