io8 



PLANT PHYSIOLOGY 



and carbon dioxide from a generator is allowed to enter the tube, either 

 from below or, as is most convenient, through the top of the tube. The 

 admission of the gas may be perfectly controlled by cautious manipulation 

 of the upper stop-cock, and this is closed at the moment when the water 



has been wholly driven out to the 

 bottom of the bore of the lower 

 stop-cock, which point is held 

 exactly at the water-level. The 

 lower stop-cock is then closed, 

 and the combination, which now 

 contains exactly the desired per- 

 centage of carbon dioxide, is 

 lifted from the water, shaken 

 free from all adhering drops, 

 and placed in position on the 

 chamber. To prevent com- 

 pression, and therefore the pres- 

 ence of too great a quantity of 

 air, in the chamber when the 

 stopper is pushed into place, 

 tiny holes (visible in the figure), 

 matching in stopper and cham- 

 ber neck, allow free release of 

 such pressure, and the chamber 

 is perfectly sealed by twisting 

 the stopper a little. The lower 

 stop-cock is then opened, per- 

 mitting the carbon dioxide of 

 the tube to diffuse into the 

 chamber, a process hastened 

 by its gravitational flow; and . 

 care should be taken to jar 

 down any water bubble which 

 may tend to close the passage 

 from tube to chamber. The 

 apparatus now contains obvi- 

 ously 2 cc. of plant and ioc cc. 

 of gas, of which a known per- 

 centage (say s, 8, or io) is carbon 

 dioxide, and the remainder is 

 air. The instrument is now 

 placed in a bright light, not di- 

 rect sunlight, for three or four 

 hours; then the lower stop- 

 cock is closed, as shown in the figure, shutting • into the tube a sample 

 of the gas of the chamber at the close of the experiment. The analysis 

 of this gas can be made at leisure, and is accomplished thus. The 



B & l. o. CO. 

 Fig. 26. — Photosynthometer; Xi- 



. Explanation in text. 



