144 PLANT PHYSIOLOGY 



cellulose itself, which makes up the cell walls of most plants, but 

 the other substances which, in the course of the maturation of 

 the cell wall, become associated with it such as lignin, cutin, 

 suberin, etc. With this in mind, cellulose may be classified as 

 follows: 



1. Normal cellulose. 



2. Compound cellulose. 



a. ligno. 



b. pecto. 



c. cuto. 



3. Reserve cellulose (also called hemi and pseudo). 



Normal or true cellulose is exemplified by that found in cotton 

 fiber, which is 99% pure cellulose, as well as that found in flax, 

 hemp, etc. The cotton fibers are freed from the small amount of 

 impurities by treating with alkali, leaving pure cellulose (C 6 Hi O5n), 

 which is a white hygroscopic substance insoluble in water, typified 

 by good paper, cotton wool, etc. When hydrolyzed by the enzyme 

 cytase it is broken down into cellobiose and then into glucose. 

 The action of certain bacteria on the cellulose teeth of the leaves 

 of Elodea, one of the water plants, can be easily seen with the 

 microscope; and it is a matter of common observation that plant 

 stems left in the ground decay in the course of time producing in 

 the soil the so-called "humins." Under stagnant water as in 

 swamps and similar places, the cellulose is broken down into car- 

 bon dioxide and marsh gas according to the equation: 



C 6 Hio05+H 2 = 3C02+3CH 4 . 



In the digestive tract of animals such fermentations sometimes 

 occur, producing in addition hydrogen, under which circumstances 

 the animal becomes " bloated"; and in 1923, Khouvine even 

 isolated cellulose bacteria from the human intestine. 



Lignocellulose is formed after the cells of the wood have reached 

 their ultimate length, and when the cellulose of the wall becomes 

 impregnated with a substance known as lignin, which gives to 

 wood its peculiar nature. The cellulose, in other words, has 

 become lignified. Cheap qualities of paper such as that in news- 

 papers are made from lignocellulose. They give the lignin tests 

 and turn yellow in the light. 



Pectocelluloses are the compounds of pectic acid and cellulose 

 previously mentioned as occurring in the cell walls of many stems, 

 storage roots, fruits, and seeds. They are about the same as the 



