130 PLANT PHYSIOLOGY 



and if now i cc. of water be placed in the bulb, there will be 2 cc. of these 

 materials and 100 cc. of air, the composition of which is of course known, 

 above the zero mark. Where greater accuracy is desired it may be attained 

 by first dropping the ten seeds into a proper measuring-glass, then filling 

 this with water to the 2-cc. mark, and finally placing both seeds and water 

 in the chamber. The index liquid to be used is now poured through the 

 reservoir tube until it stands level at the 100-cc. mark of the graduated tube 

 and at the upper index mark of the reservoir (or the lower when mercury 

 is used). The stopper, properly lubricated, is inserted with its air-opening 

 matching that of the neck, and is then .twisted, thus sealing the chamber 

 without any compression of air. The apparatus is now shielded from 

 light and placed under favorable conditions for growth. After the seeds 

 have been growing for some three or four days, an analysis of the gas is 

 made by the reagent-tube method described on another page (page 105). 

 The reservoir and rubber tube are slipped off under water, allowing the mer- 

 cury to run out, and are then used as the reagent tube, the reservoir being 

 stoppered for the purpose. Or, perhaps more simply, the determination 

 of the carbon dioxide may be made by removing the rubber tube from the 

 measuring-cylinder under a solution of caustic potash, as described on page 

 125, while the reagent tube may be applied for oxygen in the usual way. 

 The usual corrections must of course be made. The gas-pressure inside at 

 the time of reading is equalized with the atmospheric pressure by sliding the 

 reservoir tube up or down until the levels inside and out are the same, and 

 these levels are to be equalized even in case the analysis is effected in another 

 manner. For very exact work it would be necessary to take account of the 

 barometric pressures, but the slight error from this source is negligible in 

 demonstration. The temperature must either be made the same at the 

 start of the experiment and the final reading, or else, as is readily possible, 

 the change of volume due thereto must be calculated. Vapor-tension should 

 also be considered in exact work, but it is negligible in demonstration. The 

 size of the tubes is such as to eliminate all error of capillarity. After each 

 use the instrument should be thoroughly washed clear of potash. It may 

 seem an objection to a closed chamber respirometer, in comparison with 

 the open types, that the oxygen is obtained under constantly increasing 

 difficulties. But as Stich's results (Jost, 202) and my own experience 

 with this instrument show, this difficulty does not affect appreciably the 

 absorption until near the limit, and then affects only the rate. 



This quantitative study of the gases concerned in respiration 

 suggests an extension of the inquiry to the loss of substance by 

 the seed in comparison with the amount of carbon dioxide 

 formed. For an exact study of this subject the experimental 

 difficulties are great, and, besides, our knowledge of the precise 

 substances used is very deficient. Nevertheless results of some 

 value may be obtained by simple methods after the following: 



