SF.CT. ii PHYSIOLOGY 241 



mated, it will be found that it has lost a fifth of its original volume ; this means 

 that the whole of the oxygen (which makes up one-fifth of the atmospheric air) has 

 been absorbed. If caustic potash is not used in this experiment to absorb the 

 exhaled carbonic acid, the mercury remains at its original level, or, in other words, 

 the volume of air in the flask remains unchanged. From this experiment it is 

 apparent that the volume of oxygen absorbed is equal to the volume of carbonic 



COo 



acid evolved, as expressed by the formula - - 2 =l. This equivalence of volume 



U., 



between the oxygen absorbed and the carbonic acid exhaled exists only in cases 

 where the oxygen is used exclusively for complete respiration, and not where it is 

 consumed in transforming the contents of the cells, as is observed in the germination 

 of seeds rich in fat, and in the interchange of gases in the case of the succulents. In 

 the germination of seeds rich in fat, the fat is converted into carbohydrates richer 

 in oxygen. The oxygen consumed remains combined in the plant. On the other 

 hand, in the case of the succulents the production of C0 2 falls with the formation 

 of organic acids (malic acid in the case of the Cactaceae, oxalic acid in the Mesem- 

 bryanthaceae) in the dark ; these acids are, however, again decomposed in the 

 light and, according to AD. MAYER, starch is formed and oxygen set free. The 

 respiratory co-eflicient may further vary within certain limits according to the 

 nutrition and conditions of cultivation of a plant. 



The absorption of oxygen in the respiration of plants can also be shown by the 

 fact that a flame, held in a receptacle in which plants have been kept for a 

 time, is extinguished. If a lighted taper be thrust into a glass cylinder which 

 has been partially filled with flowers or mushrooms and then tightly covered 

 and allowed to remain for a day, it will be extinguished, as the oxygen of the 

 air in the cylinder will all have been absorbed. The carbonic acid exhaled in 

 respiration can be quantitatively determined from the increase in the weight of the 

 caustic potash by which it has been absorbed, or by conducting the respired carbonic 

 acid gas through baryta water and estimating the precipitate of barium carbonate. 



Intramolecular Respiration ( 58 ). In 1871 AD. MAYER showed 

 that the yeast-plant could obtain the chemical energy necessary for its 

 life from changes within its organic substance and without absorbing 

 oxygen. In the middle of the 'seventies PFLUGER made the discovery 

 that frogs are not only able to live for some time in an atmosphere 

 devoid of oxygen, but even continue to exhale carbonic acid. From 

 similar investigations it was found that the higher plants also, when 

 deprived of oxygen, do not die at once, but can prolong their life for 

 a time and evolve carbonic acid. Under these circumstances it is 

 apparent that both elements, the carbon as well as the oxygen, must 

 be derived from the organic substance of the plants themselves : the 

 oxygen can only be obtained through some unusual process of decom- 

 position carried on within the plant. This form of respiration has 

 consequently been described as intramolecular (anaerobic) respiration. 



The amount of carbonic acid produced in a given time by intramolecular 

 respiration is usually less (|-) than that given off in the same time during normal 

 respiration. There are plants, however (for instance, Vicia Faba), whose seedlings, 

 in an atmosphere of pure hydrogen, will exhale for hours as much carbonic acid as 

 in the ordinary air. During intramolecular respiration in aerobionts all growth 



R 



