18 PLANT PHYSIOLOGY 



cast off from the body. In plants, the elimination of 

 these products is more economically accomplished, for they 

 furnish the foundations for the re-synthesis of albumi- 

 nous compounds, as will be discussed under the subject 

 of nutrition (see page 71). These waste substances are 

 removed by transforming them synthetically into useful 

 compounds. 



The non-nitrogenous substances which become oxidized 

 are the fats and oils, the starches and sugars. The oxida- 

 tion may first convert the hydrocarbons into carbo-hy- 

 drates, with the liberation of energy and the formation of 

 by-products, carbo-hydrates and by-products then becoming 

 still further oxidized with the liberation of still more energy. 

 While respiration is going on, the other functions in opera- 

 tion also may involve the use, by chemical change, of some 

 of each substance produced in respiration and the formation 

 in the cell of other substances not the products of. respira- 

 tion at all. It is therefore evident that to ascertain the 

 material products of respiration is hardly less difficult than 

 to determine the amount of energy liberated. To isolate 

 any physiological process for purposes of study is impossi- 

 ble, for each process is normal only when accompanied by 

 all the processes normally going on at the same time. The 

 products of one set of chemical activities in the living body 

 may enter wholly or partially, simultaneously or succes- 

 sively, into other chemical activities. The end products can 

 be recognized and measured with comparative ease, but to 

 tell exactly where or how they are formed is much more 

 difficult and not now entirely possible. 



Water and carbon-dioxide gas are the chief products of the 

 physiological as also of other forms of combustion of car- 

 bon-containing bodies. They are formed whenever a suffi- 

 cient amount of oxygen is united with the higher carbon 

 compounds. In organisms living under such conditions that 

 the air can penetrate to all their parts, enough oxygen will 

 always be present for such complete decomposition. Under 

 ordinary conditions oxygen does not unite of itself with the 

 combustible compound, and if active (nascent) oxygen is 

 present at all in the cell it is only in amounts insufficient to 



