106 PLANT PHYSIOLOGY 



with tire tape ; and it can be compensated by carrying on the entire opera- 

 tion under water, the leakage of which does not matter. Both graduated 

 and reagent tubes may be made of any desired size, but they are more man- 

 ageable and equally accurate if made rather small, a practically convenient 

 size being that shown by the figure (holding about 8 cc), which permits 

 the absorbing solutions to be made up from sticks of commercial caustic 

 potash directly in the tubes. Thus in the reagent tube figured, the caustic 

 potash may be made up by (a) dropping 2 cm. (viz., grams) of a commercial 

 stick into the tube; (6) filling it to above the clamp with water; (c) clamping 

 it until the stick is dissolved; (d) opening it to allow escape of the air collected 

 from the stick; (e) clamping it tight again until needed. If kept closed 

 from the air it keeps good indefinitely, and if account is kept of the amount 

 of carbon dioxide absorbed, it may be used up to its limit, which is approxi- 

 mately 800 cc, though practically 200 to 400 (page 98) of that gas. For 

 oxygen absorption half the quantity of caustic potash and water is to be used, 

 and to it at the moment of using is to be added an equal amount of 1 part 

 in 15 (by weight) solution of pyrogallic acid, which should be kept closed 

 from air and from strong light. It will abscrb about 60 cc. of oxygen, say 

 25 for safety. In keeping record of the absorption it must be remembered 

 that the solutions are diluted with every use to an amount varying with the 

 amount of water in the graduated tube, an error to be guarded against. 

 These tubes may be adapted from laboratory materials, -and they are to be 

 supplied ready for use among my normal apparatus (page 46). 



The student should now make sure of his knowledge of the 

 gases available to water-plants, and their respective solubilities, 

 on which there are data in Part III. 



So far the study of the gases concerned in photosynthesis 

 has been purely qualitative. But the scientific spirit demands 

 that everything possible shall be made quantitative. To this 

 end we turn again to scrutinize the equation, which has proven 

 true so far and which may well be called, conventionally at least, 

 the pholosyrithetic equation, viz., 6C0 2 + 6H20 = C 6 Hi20 6 + 602. 

 Viewing this equation from the quantitative point of view, it 

 seems evident that the two gases concerned are equal in volume, 

 a matter which calls for experimental study. 



Are the carbon dioxide absorbed and the oxygen released in 

 photosynthesis equal in volume? 



This may be determined by keeping lighted green tissues for 1 a 

 time in a closed chamber, and then analyzing the enclosed gas for any 

 increase of oxygen and any diminution of carbon dioxide known to 

 have been present at the beginning of the experiment. Since the car- 



