334 EFFECT OF CHEMICAL AGENTS [Ch. XI 



which is most favorable for growth in later stages. So impor- 

 tant is this difference of food at different ages that agriculturists 

 persistently change the ratio of the different foods supplied as 

 their animals increase in age. The reason for this change in 

 food required lies doubtless in this, that the chemical processes 

 of growth change with the age of the animal ; at first imbi- 

 bition of water predominates, then comes the secretion of the 

 various formed substances of the organism and the constant 

 maintenance and increase of the plasma. The most favorable 

 food of an organism at any time is dependent upon the 

 metabolic processes going on at that time. 



The election of quantity is not less striking. It is well 

 known that an increase above a certain limit in the amount of 

 food presented to an organism or even actually taken into its 

 body does not result in any increase in growth. There is a 

 certain amount, fixed within broad limits, which corresponds 

 to a maximum of nutritiveness. This amount, this feeding 

 capacity, is not, however, necessarily constant at all stages of 

 the adult growth of the organism. For it has been observed 

 in certain animals, e.g. pigs, that as they grow older there is 

 a steady increase in the amount of food required to produce a 

 pound of gain in weight. Such facts serve to indicate that the 

 rate of growth is largely determined by internal factors. 



Let us now summarize the results of this study of the effect 

 of chemical agents upon the rate of growth. Of foods, those 

 used in the plastic processes are chiefly to be considered. The 

 substances serving as plastic food must contain all the elements 

 normally occurring in the organism. These are found in di- 

 verse proportions in different organisms, and hence the neces- 

 sity of dissimilar foods. Not only carbon, oxygen, hydrogen, 

 and nitrogen are necessary, but a whole series of other elements, 

 such as phosphorus, sulphur, chlorine, iodine, sodium, potas- 

 sium, calcium, iron, and magnesium are more or less essential. 

 The organic plastic food varies with the group of organisms. 

 Of relatively little importance for green plants, it becomes 

 essential to fungi. In this group we find the most important 

 character of a nutritive substance to be a certain degree of 

 lability. Among animals a mixed diet is especially beneficial, 

 but nitrogenous food favors growth more than non-nitrogenous 



