CHAPTER VIII 



FERMENTATION AND RESPIRATION 



§i. General Discussion. — -Plants grow, and in growing they produce various 

 metabolic changes and movements of materials. It thus comes about that 

 work of various kinds is performed in living plants, and this necessitates the 

 consumption of energy. The organic substances produced by green plants in 

 sunlight are sources of energy to the plant, just as wood, gasoline or coal may 

 act as the source of energy for the operation of a manufactory, the energy neces- 

 sary for the running of the machinery being supplied by the combustion of such 

 materials. The processes of living plants in which organic reserve substances 

 are oxidized by oxygen are quite analogous to combustion, and this vital 

 oxidation is known as respiration. 



The material changes that constitute respiration may be considered as con- 

 sisting typically in the absorption of oxygen and the formation of carbon dioxide 

 and water, the latter remaining in the plant body. a The general process may 

 be represented by the equation: 



Carbon 

 Glucose Oxygen dioxide Water 



C 6 H 12 6 + 6 2 = 6 C0 2 + 6 H 2 0. 



It is thus clear that these material changes of respiration proceed in a direc- 

 tion opposite to that of the photosynthetic process. Respiration results in the 

 decomposition of material by oxidation. It is really a kind of slow com- 

 bustion and, like other kinds of combustion, it is accompanied by the liberation 

 of energy. This liberated energy is used in other processes that go on within 

 the plant, or some of it may escape to the surroundings. The loss of material 

 from seeds germinating in darkness is due to this process. A part of the reserve 

 material of the seed is oxidized, and the energy thus liberated is largely used 

 in the construction of the young plant out of the remaining material. 



Normal respiration does not occur everywhere in nature. Atmospheric 

 oxygen fails to penetrate into many places where organisms may develop, as 

 in the case of stagnant water and especially in flooded soils. Hoppe-Seyler 1 

 has suggested some simple criteria for judging whether or not a soil contains 

 oxygen. Moor soil that is nearly free from oxygen is peculiarly colored. 

 Also, the formation of methane, hydrogen sulphide, ferrous carbonate and 



1 Hoppe-Seyler, Uber die Einwirkung des Sauerstoffs auf Garungen. Strassburg, 1881. P. 26.* 

 There seems to be no reason for supposing that respiration water is less apt to pass out of 

 the plant body than is water from any other source. This water must simply become a part of 

 the general water mass of the organism and the water lost by transpiration and excretion, as 

 well as that chemically fixed in photosynthesis and hydrolysis, is supplied from this general 

 mass. In this connection it may be recalled that the ordinary plant loses very much more 

 water than any other substance, during its growth. — Ed. 



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