CHEMICAL AND PHYSIOLOGICAL STUDIES. 



179 



and the resulting barium carbonate collected, washed, dried, and 

 weighed. From the weight thus obtained the volume of COj 

 evolved may be readily calculated. If, in the above experiment, 

 the gas remaining in the vessel A be tested, it will be found to 

 contain less 0^ in proportion to N than atmospheric air, thus 

 proving that oxygen has been used for the purposes of internal 

 oxidation of carbon compounds or protoplasm, and an equivalent 

 volume (or nearly so) of CO,, evolved. During respiration in 

 the plant-cell water is formed as well as carbon dioxide.* 



Fig. 114. — ApPABATDS for DeMONSTHATIXC the EvOLnTION OF CO2 



FROM A Plant during RESfiRATiON. — A, Large bell-jar ; P, plant ; 

 D, small dish containing a saturated solution of caustic potash ; 

 C, tubes containing sticks of caustic potash to absorb the COj of the 

 atmosphere before it is drawn into the jar A ; B, aspirator. 



In connection with respiration it is necessary to mention that 

 many plants possess amongst their internal tissues air-spaces 

 and intercellular spaces which are filled with oxygen or atmo- 

 spheric air. These spaces form at times channels of aeration of 

 considerable extent, more especially in such plants as Nymiohcea, 

 Nwphar, and Bquisetiim. The value of such aeration is manifest, 

 in that the oxygen needed for respiration surrounds masses of 



* Some plants (anaerobic bacteria) are able to exist in the absence of 

 free oxygen, and in them a process known as intramolecular respiration 

 goes on. 



