NUTRITION. 163 



line of demarcation. Popularly the term putrefaction is ap- 

 plied to the changes in nitrogenous substances which are ac- 

 companied by offensive odors. Fermentation is commonly 

 applied to the chemical changes occurring in sugary solutions, 

 such as fruits, expressed juices, infusions, etc. Many bacteria 

 and a number of fungi, notably those known as yeasts, are 

 capable of producing fermentation in such solutions. The 

 chemical changes produced are more extensive than those 

 required for obtaining food. Ordinary brewer's yeast, for 

 example, utilizes about 5 per cent of the sugar present in the 

 solution for food, but breaks up the remaining 95 per cent 

 into carbon dioxide, alcohol, and some other less important 

 by-products. In putrefaction the by-products are commonly 

 offensive gases, among which hydrogen sulfid (H,S) predomi- 

 nates. Various other materials may be formed, among which 

 not infrequently are virulent poisons. These are well known 

 in certain putrefactive changes of milk, meat, etc. 



222. Parasites obtain their food either by growing upon 

 the surface of the host and thrusting into its interior absorb- 

 ing organs ; or by growing wholly in the interior of the host, 

 breaking out to its surface only to form reproductive bodies. 



Parasites may work little apparent harm, or they may bring 

 about local disease and death of the host. Their mode of 

 obtaining food is not essentially different from that of sapro- 

 phytes. They either digest solid foods, or absorb liquid 

 foods, prepared by the host for its own use. Among the 

 green plants there are some partial parasites, such as the mis- 

 tletoe, which seem to obtain from their hosts chiefly the 

 water and salts which they have absorbed. These materials 

 they themselves elaborate into food. (See further ^f 465.) 



D. Nutrition of green plants. 



223. Raw materials. — In order that the green plants may 

 be able to manufacture their food, they require certain raw 



