RESPIRATION. 367 



RESPIRATION. 



967. It has been long known that air is necessary for the 

 germination of seeds. 1 In 1777 Scheele 2 pointed out that in this 

 process, as in the breathing of animals, oxygen (called by him 

 fire air) is consumed and carbonic acid (called by him air-acid) 

 is given off. Two years later, Ingenhousz 8 showed that all 

 plants at night give off fixed air (carbonic acid), and in 1804 

 Saussure proved that all plants require oxygen for their growth. 

 In 1838 Meyen 4 clearly defined the scope of respiration in plants, 

 since which time it has been carefully examined in most of its 

 relations. 



968. The relations of gases to plants, so far as their absorp- 

 tion and elimination are concerned, have been sufficiently dis- 

 cussed in Chapter X. It is merely necessary to state at present 

 that oxygen is readily absorbed by all parts of plants, and that 

 the intercellular passages (519) form a means by which it can 

 traverse the whole plant very rapidly. 



969. In its simplest phases respiration consists in the absorp- 

 tion of oxygen, the oxidation of oxidizable organic matters, and 

 the evolution of the products 



of oxidation; namely, carbonic 

 acid and water. Some other 

 products are often formed in 

 minute amount, but these may 

 be here disregarded. 



970. Measurement of Res- 

 piration. Respiration can be 

 measured very nearly by the 

 amount of oxygen which dis- 

 appears or by the amount of 

 carbonic acid which is given 

 off. The ordinary apparatus 

 for examining respiration is 

 based upon the measurement 

 of the latter, and consists es- 

 sentially of some application of potash-bulbs, or wash-bottles (see 

 Fig. 163), for the interception of all evolved carbonic acid. The 



1 See Malpighi : Opera omnia, 1686. 



2 Chemische Abhandlung von der Luft, 1777. 



8 Experiments upon Vegetables, 1779, p. xxxvi. 

 Pflanzenphysiologie, ii., 1838, p. 1C2. 



