126 PLANT PHYSIOLOGY 



ble in elementary work, though attention should be called to it. But a more 

 serious fallacy occurs in connection with the familiar make-shift experiment 

 in which a candle or other flame lowered into a vessel containing seeds which 

 have been germinating for some hours, becomes extinguished. By some 

 this is taken as evidence that carbon dioxide has been formed, by others 

 that oxygen has been removed, and by others that both of these things have 

 happened. In fact it may be due to either of these things or to the forma- 

 tion of some other non-combustion-supporting gas, and taken alone, unsup- 

 plemented by other evidence, it is wholly illogical and inconclusive. Some 

 loose or unfortunate ways of approaching the teaching of Respiration are 

 discussed by Shaw in Science, 25, 1907, 627. 



The release of carbon dioxide in conjunction with loss of 

 weight shown by this experiment at once recalls the reciprocal 

 phenomenon already studied in photosynthesis, and this in turn 

 must raise the query whether the parallelism extends farther 

 and includes an absorption of oxygen. Hence we have to deter- 

 mine: 



75 oxygen absorbed by the working tissues 0} plants? 



This may readily be settled by a test of the gas remaining in a 

 chamber after the carbon dioxide formed by working tissues has been 

 removed. 



Experiment. After the application of the test for carbon dioxide 

 to the respiroscope tubes of the preceding experiment, test the same 

 for the presence of oxygen; this may most conveniently be done by 

 use of the reagent tubes. 



Demonstration Methods. The disappearance, whole or partial, of 

 the oxygen may most effectively be demonstrated by using the normal respi- 

 rometer later described (page 129). A special arrangement for the purpose 

 is described by Detmer in his "Kleines Praktikum," 139. The absorption 

 of oxygen is, however, usually demonstrated indirectly by showing that all 

 activity (growth, etc.) stops when none of that gas is available. Oxygen 

 may be excluded by any one of several methods, controls being needed in 

 all cases, as follows, (a) Germinating seeds of measured radicle length are 

 placed on wet filter-paper in the bottom of a wide bottle having a very tight 

 stopper pierced by a slender glass tube; for this a wide form of chemical 

 wash-bottle is good, or a calcium-chloride tube may be found to nearly fit 

 the neck of a bottle or conical flask where it may be made tight with sealing- 

 wax. The air is then exhausted to a vacuum, and, while the exhaust is on, 

 the tube is sealed in a Bunsen flame. After some days of good growth con- 

 ditions, the bottle is reopened (by breaking the sealed tip) and the radicles 

 are remeasured. The tightness of the joints can be assured by added sealing- 

 wax, and a small mercury manometer should be inserted to demonstrate the 

 vacuum. (6) An ingenious method of securing the exposure of seeds to a 



