METABOLISM 125 



Respiration. — Hitherto we have been considering those physio- 

 logical changes which involve the progressive building up and 

 elaboration of organic materials, a constructive process which 

 reaches its climax in the production of protoplasm. This 

 constant upbuilding and renewal of the living substance is suc- 

 ceeded by an equally constant process of disintegration, which 

 results in the liberation of energy and which is usually accom- 

 panied by the intake of oxygen and the outgo of carbon dioxide. 

 To this general process the name respiration has been given. 



Before we enter upon a detailed study of this important phase 

 of plant physiology we should discuss briefly the problem which 

 it brings up, namely the energy-relations of the plant, of which 

 the processes of food synthesis and nutrition form an essential 

 part. Like every living thing, the plant is continually active. 

 This activity shows itself in movement of various sorts, either 

 of the plant body as a whole, of the substances within it, of the 

 atoms and molecules during those chemical changes which are 

 always taking place in living cells, or in the phenomena of 

 growth. These various movements, the maintenance of which 

 is necessary if the plant is to remain alive, require the expenditure 

 of energy, as do any movements of matter; and one of the chief 

 problems in the economy of the plant, as in the operation of a 

 machine, is to obtain an adequate supply of energy and to liberate 

 it at the proper times and in the proper places. 



Kinetic and Potential Energy. — Energy exists in the universe 

 in two forms: Active or kinetic and stored or potential energy. 

 Kinetic energy performs work by setting matter in motion, 

 sometimes by changing its position, sometimes by raising its 

 temperature, sometimes by producing chemical alterations 

 within it, and sometimes in other ways. Potential energy is 

 inactive energy, stored up in an object by virtue of the position 

 or condition of that object. Potential energy exists in a stretched 

 spring, in a bent bow, in the water of a mountain stream, in a 

 charged battery, in a piece of coal, or in an explosive. It is 

 present in an object only as the result of the previous expenditure 

 of kinetic energy upon that object. Care should of course be taken 

 not to confuse this "storage" of energy with the storage of food 

 or any other form of matter. The presence or absence of a supply 

 of stored energy in a given body merely affects the relations 

 between its parts and does not alter in the least the Inilk of the 

 object or the amount of matter which it contains. We need 



