DIV. ii PHYSIOLOGY 269 



The Ripening of Succulent Fruits. A striking transformation of substances 

 takes place in the ripening of succulent fruits. The relatively rare case of the 

 formation of fats has already been mentioned. Much more frequent is the change 

 of starch into sugar associated with the disappearance of organic acids and tannins. 

 The fruits thus become sweet-tasted instead of acid or bitter, and are eaten by 

 animals which distribute the seeds. The significance of these chemical changes 

 is thus ecological. 



V. Respiration and Fermentation 



In the higher plants all 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 with the absorption of oxygen and is termed 

 respiration, does not lie in the substances formed but in the libera- 

 tion 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 without being first assimilated. The decomposi- 

 tion may be effected by oxidation, reduction, or dissociation ; all these 

 processes are grouped togejther 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. RESPIRATION 



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. 244). 



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 off carbonic acid. The question had already 

 been thoroughly investigated by SAUSSURE, and by DUTROCHET in 

 the years 1822 to 1837, and its essential features correctly interpreted. 

 Later the existence of respiration in plants was doubted owing to 

 the demonstration of their power of decomposing carbon dioxide and 

 giving off oxygen ; it seemed impossible that both processes could go 

 on at the same time. The correct view was then formulated by 

 SACHS. ASSIMILATION AND RESPIRATION ARE TWO DISTINCT VITAL 

 PROCESSES CARRIED ON INDEPENDENTLY BY PLANTS. WHILE IN THE 



PROCESS OF ASSIMILATION GEEEN PLANTS ALONE, AND ONLY IN THE 



