II. HISTORICAL PART. 

 Introducticn. 



According to the current practice of agricultural analysts, the car- 

 bohydrates of plants are reported as crude fiber and nitrogen-free 

 extract. Crude fiber is the term applied to the resistant mixture form- 

 ing the mature cell wall, shown as long ago as 1864 by Henneberg and 

 Stohman (41) to have no definite chemical composition. It is there- 

 fore not identical with cellulose, but consists of a mixture of cellulose 

 with incrusting substances, lignin and cutin, the relative proportions 

 of which have recently been exhaustively studied by Konig (51), 

 Fiirstenberg (39), and Murdfield (63). Cellulose is the chief consti- 

 tuent; the other two are usually present in varying proportions. 



Schulze (74) to whom much of our knowledge of the composition of 

 the plant cell wall is due, has classified the carbohydrates of the nitro- 

 gen-free extract as follows : 



I. Water-soluble carbohydrates. To this class belong the mono-, 

 di-, and tri-saccharides, and some soluble polysaccharides. 

 II. Carbohydrates insoluble in water, but yielding sugar under 

 the action of diastase. The chief member of this group is 

 starch. 

 III. Carbohydrates insoluble in water and resistant to the action 

 of diastase, never being changed by it into sugar. This 

 group is called the Hemicelluloses. 



The term hemicellulose, as used by recent writers^ seems to be inter- 

 preted to include some polysaccharides of the first group. It is there- 

 fore used here as a group name for those carbohydrates which are dis- 

 tinguished from cellulose by being capable of hydrolysis on boiling 

 with dilute mineral acids, and from the other polysaccharide carbohy- 

 drates by not being readily digested by diastase. According to the 

 kind of sugar yielded on hydrolysis, the hemicelluloses are designated 

 as Pentosans or Hexosans, the latter including Galactans, Mannans, 

 Dextrans, Levulans, etc. After a general review of the chemical 



^e.g., Lohrisch. 



259 



