102 



PLANT PHYSIOLOGY 



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siderable capacity. Place in the jar, with the cut ends in the test- 

 tube, vigorous shoots of Cabomba, Elodea, or other plants growing 

 habitually submersed, and stand the apparatus in a moderate (never 



intense) light for some hours. 

 When sufficient gas has collected 

 in the tube, apply to it an oxygen 

 absorbent, and determine how 

 much, if any, of the gas is oxygen. 



Precautions. The plants should 

 be in growing, not resting, condition. 

 Such plants as give off the gas freely 

 through their cut stems are best 

 treated as described above, but with 

 some others, which give off the gas 

 from their leaves, it is necessary to 

 use a funnel inverted in the water 

 and capped by the test-tube, as 

 shown in all books. In this case, 

 however, it is indispensable that the 

 funnel be not allowed to rest on or 

 near the bottom, and that it shall not 

 fill the vessel, as otherwise the access 

 of carbon dioxide from the atmosphere 

 through the water to the plants is 

 cut off, and gas soon ceases to rise. 

 Also, the gas will be yielded much 

 better if the water is first charged 

 with carbon dioxide from a generator, 

 or from a seltzer bottle, though it will 

 not do to add it in any quantity after- 

 Fig. 24. — Efficient arrangement for wards, since, as Pantanelli has 

 study of the release of oxygen shown, if the supply is too large the 

 in photosynthesis by water- gas will pass ty diffusion through the 

 plants; Xi- plant hto the test-tube. In making the 



test for oxygen, the flaming of a glowing 

 splinter often fails because of the lowness of the percentage of oxygen (com- 

 pare Pfeffer, 1, 331), and it is better to use either the phosphorus test or 

 the pyrogallate-of-potash test, described below. The optimum of lijzht for 

 the process depends somewhat upon the amount of carbon dioxide available, 

 but is always far below full sunlight. 



Other Methods. Several ways of applying the above method with 

 convenience and celerity have been described, notably by Hansen in Flora, 

 86, 469, and by Linsbauer in the same, 97, 1907, 263, and by Stone in Tor- 

 reya, 4, 1904, 17. A self-registering form of this apparatus, liable however 

 to serious practical errors from temperature changes, is described by Cope- 

 land in the Botanical Gazette, 29, 1900, 440. Pfeffer, in his paper earlier 



