48 PRINCIPLES OF ANIMAL BIOLOGY 



phenomena should be inchided in the definition. From the brief ac- 

 count of the activities of protoplasm outlined above it is obvious that 

 metabolism may be divided into two phases, namely, the constructive 

 phase or anabolisrn and the destructive phase or catabolism. Anabolism 

 includes all the processes concerned in the growth and repair, or up- 

 building of protoplasm. It properly includes all processes of synthesis 

 by which substances are transformed into reserves of food. Catab- 

 oUsm, on the contrary, includes all those processes opposed to 

 anabolism. These are the processes by which protoplasm is broken 

 down and the waste products eliminated. Both anabolism and catab- 

 olism are continuous processes and both are of great importance. As 

 long as anabolic processes are in excess of catabohc processes growth 

 continues; but when catabolic processes are in excess the reverse of 

 growth takes place. Thus in progressive emaciation due to starvation 

 or under-nutrition catabolism is in excess of anabolism, reserves of food 

 (carbohydrates and fats) are being consumed, and body weight decreases. 

 It is a relatively easy matter to define the two phases of metabohsm 

 and to use the terms anabolism and catabolism in a general way, but 

 it is a more difficult matter to apply the terms to specific processes. 

 Some of the aspects of metabolism barely touched upon in the foregoing 

 discussion are considered further. 



Intake and Preparation of Substances. — Since the material intake of 

 plants and that of all but a few simple animals is very different, and 

 since animals are wholly dependent directly or indirectly upon plants 

 as a source of food, the intake of substances in plants is considered first, 

 although it is not desirable to separate from it entirely the subject of 

 animal foods. 



Intake in Plants. — Something in regard to the materials which plants 

 take in may be learned by finding out the elements which compose their 

 bodies, and the materials which they give off. Analysis of the dried 

 substance of green plants shows that they always contain carbon, hydro- 

 gen, oxygen, and nitrogen. From the ash or burned bodies of plants 

 sulphur, phosphorus, potassium, magnesium, calcium, iron, sodium, 

 chlorine, and silicon are always recovered. Iodine, bromine, fluorine 

 and many other elements may be found in the ash at times. All the 

 elements used by the plant must be secured from the air, from water, and 

 from the soil solution or the solution (for example, sea water) in which the 

 whole plant is immersed. By experiments in which higher green plants 

 are grown in water cultures containing known substances in solution it 

 has been determined that these plants have a certain minimum chemical 

 requirement and that if any of the required substances are not available 

 normal development is impossible. These necessary elements are car- 

 bon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron. Except oxygen, these elements are never 



