COIMPARATIVE RATES OF RESPIRATION 513 



ism. During the oxidation of foods a number of highly reactive intermediate 

 compounds are produced in plant cells. The chemical nature of some of these 

 substances will be discussed in the next chapter. These highly labile com- 

 pounds apparently take part in numerous reactions leading to the synthesis 

 of various types of more complex compounds w^hich are essential constituents 

 of the living system of plant cells. 



Methods of Measuring Respiration. — Respiration rates are measured in 

 terms of the rate of oxygen consumption, or the rate of carbon dioxide evolu- 

 tion, or both. Rates of carbon dioxide release are more commonly determined 

 than rates of oxygen consumption, since the chemical and physico-chemical 

 methods of detecting changes in the rates of carbon dioxide evolution are 

 easier to work with. Respiration rates are often determined by enclosing 

 the plant in a suitable chamber through which air is allowed to flow at an 

 appropriate rate. The carbon dioxide in the effluent gas stream can be pre- 

 cipitated as BaCOs by bubbling the gas through a solution of Ba(OH)2. 

 The quantity of this compound formed can be measured either volumetrically 

 {i.e. by titration), gravimetrically (by determining the weight of BaCOs 

 precipitate formed), or by measuring the rate of change of electrical con- 

 ductivity of the Ba(OH)2 solution. It is also necessary to remove the carbon 

 dioxide from the gas stream before it passes through the plant chamber or 

 else to make a check analysis of its carbon dioxide content. If it is desired 

 to determine carbon dioxide liberation and oxygen consumption simultaneously, 

 the simplest procedure is to collect the gas stream in a reservoir after it has 

 passed through the plant chamber and analyze it for the proportions of these 

 two gases present. If the volume of each of these gases which has passed into 

 the plant chamber is also known the gaseous exchanges of the plant can be 

 computed. 



Rates of respiration are expressed in terms of either carbon dioxide evolu- 

 tion or oxygen consumption per unit of time, and are usually calculated on the 

 basis of a unit dry weight of tissue. Obviously accurate determinations of 

 respiration rates of chlorophyllous plant organs can only be obtained if they 

 are enclosed in a respiration chamber which is impervious to light. 



Comparative Rates of Respiration. — Rates of respiration as expressed 

 either in terms of oxygen consum.ption or carbon dioxide liberation vary greatly, 

 depending upon the plant organ or tissue. Since the seat of respiration lies 

 in the protoplasm a correlation often exists between the proportion of pro- 

 toplasm present in a tissue and the intensity of the respiration process in that 

 tissue. As a general rule respiration rates are found to be greatest in meriste- 

 matic tissues such as growing root or stem tips or the embryos of germinating 

 seeds. It is precisely in such tissues that the proportion of protoplasm is 



