SECT. II PHYSIOLOGY 241 



V. Respiration and Fermentation 



In the higher plants call the organic substance produced in 

 assimilation is not used for construction and storage purposes ; 

 a part of it is always broken down and returns to the state of 

 inorganic compounds. The significance of this process, which is 

 usually associated \\dth the absorption of oxygen, does not lie in the 

 substances formed but in the liberation of energy which is essential 

 for the life of the plant. In certain lower plants the necessary supply 

 of energy may be obtained in other ways. Usually organic substances 

 are absorbed from the substratum and broken down Avithout being 

 first assimilated. The decomposition may be effected by oxidation, 

 reduction, or dissociation ; all these processes are grouped together 

 as fermentation. Other lower organisms can utilise the energy set 

 free in the oxidation of certain inorganic compounds. Transitional 

 forms occur between the various methods of obtaining the necessary 

 energy. 



A. Eespiration 



By respiration in its typical form is understood the oxidation of 

 organic material to carbon dioxide and water ; this involves the 

 absorption of oxygen from without (cf. p. 211). In the higher 

 animals the process of respiration is so evident as not easily to 

 escape notice, but the fact that plants breathe is not at once so 

 apparent. Just as the method of the nutrition of green plants 

 was only discovered by experiment, so it also required carefully 

 conducted experimental investigation to demonstrate that PLANTS 

 ALSO MUST BREATHE IN ORDER TO LIVE ; that, like animals, they 

 take up oxygen and give oft' carbonic acid. Although the question 

 had already been thoroughly investigated by Saxjssure, and by 

 DuTROCHET in the years 1822 to 1837, and its essential features 

 correctly interpreted, LiEBiG pronounced the belief in the respiration 

 of plants to be opposed to all facts, on the ground that it was 

 positively proved that plants on the contrary decomposed carlionic 

 acid and gave off the oxygen. He asserted that it was an absurdity 

 to suppose that both processes were carried on at the same time ; and 

 yet that is what occurs. 



Assimilation and respiration are two distinct vital pro- 

 cesses CARRIED ON INDEPENDENTLY BY PLANTS. WhILE IN THE 

 PROCESS OF ASSIMILATION GREEN PLANTS ALONE, AND ONLY IN THE 

 LIGHT, DECOMPOSE CARBONIC ACID AND GIVE OFF OXYGEN, ALL PLANT 

 ORGANS WITHOUT EXCEPTION BOTH BY DAY AND BY NIGHT TAKE UP 



OXYGEN AND GIVE OFF CARBONIC ACID. Organic substance, obtained 

 by assimilation, is in turn lost by respiration. A seedling grown in 

 the dark, so that assimilation is impossible, loses by respiration a con- 

 siderable part of its organic S'lbstance, and its dry Aveight is consider- 

 ably diminished. It has been found that during the germination of 



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