422 The Physiology of Plants BOOK in 



separate respiratory material, consisting for the most part 

 of carbohydrates and fats, or of groupings of them in the 

 protoplasmic molecule, nevertheless maintained its hold 

 on the opinion of many observers and cannot even now 

 be said to have disappeared, although the view of a direct 

 combustion, apart from the interaction of the living sub- 

 stance, has been to a certain extent modified by those 

 who advocate it. 



The weakness of the combustion hypothesis became 

 apparent when the relations of the two gases concerned 

 in the interchange came to be scrutinized. The animal 

 physiologists took the lead in such investigations, but their 

 results were soon supported by the work of botanists. 

 Wolkoff and Mayer showed in 1874 that something more 

 than mere combustion is involved, by the discovery that 

 raising the temperature of the plant at first accelerates 

 the respiratory interchange, but that after a certain tem- 

 perature, varying with the plant, is reached, further 

 elevation depresses it. Ziegenbein, in 1893, also demon- 

 strated the existence of an optimum temperature for respira- 

 tion; he placed it at about 35 C. Deherain and Moissan, 

 in 1874, investigated the intake of oxygen and the output 

 of carbon dioxide separately, and found that variations 

 of temperature affect them differently ; at a low tempera- 

 ture the absorption of oxygen is more active than the 

 exhalation of carbon dioxide, while at a high temperature 

 the contrary is the case. During the early seventies Pfliiger 

 and other physiologists showed the independence of the 

 two processes by discovering that frogs kept at a low 

 temperature for several hours in the absence of oxygen 

 continued to exhale carbon dioxide. A somewhat similar 

 observation was made by Wortmann in 1880 ; he showed 

 that a germinating seed placed in a Torricellian vacuum 

 continually exhales carbon dioxide. 



The independence of the two processes became more 



