6 FEEDS AND FEEDING 



number of molecules of glucose. It is the common structural substance 

 of plants, forming the chief part of the walls of the cells of which they 

 are composed. The thickness of the cell walls, and consequently the 

 percentage of cellulose, varies greatly in different parts of plants, the 

 walls being thick and resistant in the woody stems, and thin and delicate 

 in the softer parts, such as the fruits and leaves. Especially in the 

 woody parts of plants, the cell walls do not consist merely of pure 

 cellulose, but of cellulose combined with other related carbohydrates, 

 which are even tougher and more resistant. In analyzing plants the 

 chemist includes cellulose and these other compounds under the term 

 fiber. 



The pentoses are sugars which are similar to glucose and fructose but 

 have 5 atoms of carbon in the molecule, in place of 6. The pentoses 

 themselves do not commonly occur in plants, but the pentosans, which 

 are formed from them in the same manner as starch is formed from 

 glucose, are widely distributed. Pentosans are found in largest amounts 

 in the more woody portions of plants and in the outer portions of seeds. 

 While corn grain contains less than 6 per ct. of pentosans, usually 

 straw and hay from the grasses contain over 20 per ct. and wheat bran 

 about 24 per ct. 



10. Fats and oils. In some cases the plant stores reserve food in the 

 form of fat, which is solid at ordinary temperatures, or of oil, which is 

 liquid. Such storage is entirely possible, since fats and oils are formed 

 from the same elements that make up the carbohydrates ; i. e., carbon, 

 hydrogen, and oxygen. Fats and oils are commonly included under 

 the general term " fats'*. 



Fats are formed in plants by the union of compounds called fatty 

 acids with glycerin. The proportion of oxygen is much less, and of 

 carbon and hydrogen much greater than in the carbohydrates, as the 

 following formulae of three common vegetable fats show: 



Stearin C 5T H 110 6 Palmitin C 51 H 98 6 Olein C 57 H 104 6 



Due to the fact that they contain a larger proportion of carbon and 

 hydrogen than the carbohydrates, the fats give off more heat on burn- 

 ing, one. pound of fat producing about two and a quarter times as much 

 heat as a pound of carbohydrates. Oils and fats most abound in the 

 seeds of plants, the reserve food supply in peanuts and flax seed, for 

 example, being stored largely in this condensed form. When seeds con- 

 taining oil, such as the flax seed, begin to grow, the oil is changed over 

 into products which nourish the growing plantlet the same as is done in 

 ordinary seeds by the sugar which is formed from the stored starch. 



Under the general class of fats there are included, in addition to the 

 true fats or oils, other compounds somewhat similar to the fats in their 

 properties. Among such compounds are the lecethins (phosphorus con- 

 taining substances) and the waxes. These are present in but small 

 amounts in feeds and need no special consideration in the. feeding of stock. 



