460 PROTOPLASM 



Associated with cellulose, in a manner similar to that just 

 described for pectin, are numerous other compounds generally 

 regarded, like the hemicelluloses, as derivatives of cellulose. 

 Among them are the gums, mucilages, and gelatinous substances, 

 usually produced during heartwood formation. Their origin and 

 chemical constitution are not well understood. 



Cellulose is almost wholly a plant product, yet, like most 

 features used to distinguish plants from animals, it is not an 

 infallible criterion of what is a plant and what an animal. Tuni- 

 cates and insects are reported to have tunicin in their tests or 

 pellicles. This substance is said to be identical with cellulose. 



While cellulose is used primarily by the plant as a material for 

 wall building, it may serve, probably in some modified form, as a 

 reserve food. Cellulose is also food for certain animals which, 

 though lacking the capacity to digest it themselves, are never- 

 theless able to use it because of their intestinal flora. There is 

 no digestive enzyme in the fermentation fluids of higher animals 

 that will act upon cellulose, nor indeed is any intestinal ferment 

 known that will attack the hemicelluloses, the pentosans, or the 

 galactans, yet these last two carbohydrates certainly, and 

 probably some of the higher celluloses, not only are utilized by 

 animals but form an important part of the dietary of herbivora. 

 This is possible because the digestion of the cellulose is carried 

 out by microorganisms. It is said that the intestinal juices of 

 the horse dissolve 70 per cent of favorable nonlignified cellulose 

 but that the ferments are produced by bacteria or Protozoa. 

 The cow is another example of a higher animal that digests 

 cellulose. In all such cases, the fermentation is done by micro- 

 organisms. The digestion products apparently are not monosac- 

 charides, as one would expect, but carbon dioxide, methane, and 

 fatty acids, the last only being suitable for nutrition. (The use 

 of cellulose as food should not be confused with its important 

 function in diet as bulk, or "roughage," promoting peristalsis.) 



The classical example of the wood-feeding habit in animals is 

 that of termites. Intestinal Protozoa make it possible for these 

 insects to live on wood. When defaunated (robbed of their 

 protozoan companions) by heat or oxygen, they cannot digest 

 wood and die from starvation when fed it, but they can then 

 live on rotted wood, that is to say, wood predigested by fungi. 

 If intestinal Protozoa of the same kind as were removed are 



