IOO 



PLANT PHYSIOLOGY 



when under 3% strong, puts out the light, which will only bum again after 

 the carbon dioxide is removed. It may be applied as follows. Select a 

 wide-mouth large bottle, and prepare for it a good gas-tight flat stopper 

 having in its middle an opening 1 an. in diameter closed by another stopper 

 (Fig. 23); attach a candle, the smallest obtainable, at the end of a bent 

 wire; holding the wire with the bend under water, light the candle and 

 place the bottle inverted over it with the mouth in the water. Count the num- 

 ber of seconds until the candle goes out; then remove it and insert under 

 water a large shoot of a green plant; slip a saucer under the whole, lift it 



out and stand it for a day in bright 

 diffused light; at evening transfer it 

 back to the water, remove the saucer 

 and plant, insert, under water, the double 

 stopper, and invert the bottle with 

 its small quantity of contained water. 

 The wire is now bent of a form such 

 that the candle may be lowered to the 

 center of the jar, and is then pushed 

 through a cork of the same size as that 

 closing the bore of the larger stop- 

 per. The smaller cork is now quickly 

 removed, and the candle is lowered into 

 the jar, the cork on its wire tightly 

 closing the opening. The number of 

 seconds is now counted until the flame 

 goes out, a comparison of which with 

 the similar test of the morning gives 

 a crude measure of the amount of 

 the carbon dioxide which has been 

 absorbed. In some ways better than 

 a candle is a tiny gas-flame, which can 

 be made much smaller than that of 

 any obtainable candle, and hence heats 

 and expels the air from the jar to a less degree, besides permitting a more 

 accurate count. The jet can be constructed of glass tubing drawn to a capil- 

 lary point, the tubing being put through a cork as shown by Fig. 23. In 

 order that the flame may be the same size at the morning and evening tests, 

 the arrangement figured, in which the point may be swung on a movable 

 rubber connection, should be used. The gas-supply must be cut off the 

 moment the flame goes out (by pinching the rubber connection-tube). The 

 experiment is liable to obvious errors (absorption of some carbon dioxide 

 by the water, some gas-exchange at the insertion of the candle, etc.), but 

 nevertheless gives results correct in the main. A method of avoiding the 

 latter error, by lighting the candle inside the jar, is described by MacDoucal 

 in his " Elementary Plant Physiology," 93. The experiment is much more 

 effective, especially for younger students, if a second bottle is stood beside 

 the first and treated like it in every respect except that it is covered from 



Fig. 23. — Arrangement foe a 

 very simple demonstration of 

 the absorption of carbon diox- 

 ide by green tissues; x |. 



The second figure shows use of a gas- 



flame instead of a candle, 

 explanations in text. 



Further 



