12 THE CHEMISTRY OF CATTLE FEEDING 



necessary, and it has never become customary to make them 

 in practice. Under ordinary conditions, the natural food and 

 drinking water contain enough phosphates and lime to satisfy 

 all the requirements of the animal. 



Besides phosphate of lime, bones contain a small quantity 

 of magnesium and fluorine. Together, these two substances 

 amount to little more than i per cent, of the ash, and it is 

 not known with certainty in what form they are present or 

 what functions they perform. In view of the regularity of 

 their occurrence, their presence cannot well be regarded as 

 purely accidental or superfluous. 



Silica is a normal constituent of all plants. The fact is 

 probably a necessary consequence of the presence of soluble 

 silicates in the soil. Of cultivated plants, the grasses and 

 cereals contain the largest amount. Possibly because they 

 have a greater capacity for assimilating such compounds. 

 Silica is found in largest quantity in the stems and leaves 

 and in the husks of grain always in the exterior portions 

 and is regarded by botanists as of the nature of an excretum. 

 It is never concentrated in the regions of active growth or 

 stored up with embryonic nourishment. It forms a protective 

 covering for seeds and perhaps increases the rigidity of straws 

 and stems, but, apparently, it is not indispensable for the 

 growth of the plant. 



The presence of silica in animals is practically limited 

 to wool, hair, feathers and similar epidermic structures. It 

 occurs only in insignificant quantities and its presence is 

 probably accidental. The silica 1 in the food consumed by 

 animals is excreted unchanged, and it is safe to say that it has 

 no nutritive value. 



Oxide of iron occurs only in very small proportions in the 

 ash of plants, but is found in more considerable quantities in 

 animals. It is probably an essential constituent of both. 

 Certain protein compounds of the blood and possibly also of 

 the muscular tissues contain iron as an essential ingredient. 



To sum up, it may be said that of the ingredients of the 



1 N.B. The term "silica" is not synonymous with sand, though it i s 

 often said to be. 



