128 PLANT PHYSIOLOGY 



cause of the clearer definition and possible extension of knowl- 

 edge such a treatment will give, but also in order to follow farther 

 the suggestive parallelism with photosynthesis. Accordingly we 

 face this problem: 



What quantitative relationships exist between the gases ex- 

 changed in respiration? 



This may be determined, of course, by accurate quantitative analy- 

 sis of the air which, for an interval, has surrounded germinating seeds 

 or other working tissues. 



Experiment. Germinate some good oats or barley grains until 

 the roots are visible (2-4 mm. long). Select about ten of these and 

 determine their volume (by immersion in water in a small measuring- 

 glass) ; place them with some moisture in the chamber of a respi- 

 rometer, the measuring-tube of which contains a solution of caustic 

 potash; give favorable conditions for growth and observe the rise 

 of the solution; as soon as this has reached its limit, apply the test 

 for oxygen and compare the volumes concerned. 



Precautions. The experiment must not be allowed to continue after 

 the oxygen is absorbed, for then decay gases are released, to the detriment 

 of the result. Of course the usual corrections for temperature, etc., must 

 be applied, and care must be taken that the caustic potash cannot diffuse 

 up streams of moisture to the seeds. 



Respirometers. The various respiroscopes already described may also 

 serve as respirometers if properly used, viz., with the mercury level started 

 some distance up the tubes, with allowance for volume occupied by the seeds 

 and moisture, with proper corrections for temperature, etc., and with an 

 accurate measurement of the volumes occupied by the absorbing liquids 

 used in the tests. But of respirometric methods proper there are several. 

 Thus the living tissues may be kept for a time in a closed chamber with a 

 carbon -dioxide-absorbing substance, whose carbon -dioxide content is later 

 determined, as in Stich's apparatus (Detmer, 278); or a sample of the 

 gas may be withdrawn for analysis by methods which Richards has espe- 

 cially refined (Annals of Botany, 10, 1896, 531). Richards has also described 

 a suitable small chamber for seeds (Torreya, 3, 1903, 113). The commonly 

 used method for exact work, which measures, however, only the carbon 

 dioxide released, is that of Sachs (described in Detmer, 259, and in Dar- 

 win and Acton, 3), in which air freed from carbon dioxide is drawn by an 

 aspirator over germinating seeds into tubes of baryta water, the amount 

 of the carbon dioxide being subsequently determined by titration. This 

 method has ' been improved by others, notably by Blackman, whose 

 apparatus is the most precise for this purpose yet constructed (Annals of 

 Botany, 9, 1895, 161). The Pettenkofer baryta tubes, much used in the 

 analysis of the gas, are supplied by Arthur in his apparatus (Botanical 

 Gazette, 22, 1896, 468). A great simplification of the apparatus for demon- 



