CH. Il] GAS EVOLVED. 37 



intensity of illumination may be easily varied by placing 

 the light at various distances from the plant. The 

 variations in the rate of escape of gas do not however 

 seem to be proportional to the changes in the intensity of 

 light, as WolkofF 1 found to be the case when diffused light 

 was used for illumination 2 . 



(43) Dependence on CO Z . 



Transfer the plant to a beaker filled with water which 

 has been boiled in the apparatus shown in fig. 7, when 

 the rate of bubbling immediately falls off greatly. 



After a time the water may be supplied with CO.., by 

 blowing vigorously into it through a glass tube, when the 

 evolution of gas increases in amount. As a check on the 

 result the beaker should finally be placed in the dark, to 

 make sure that the increased rate of bubbling is not a 

 physical effect like that produced by effervescent water. 



(44) Temperature. 



Provide two beakers of water, one at a temperature 

 of 24 26 C., the other at 4 5 C. Place a specimen 

 in the warmer of the two and when the readings are 

 constant transfer it to the cold water. In some of our 

 experiments on Elodea we found that the escape of gas 

 was immediately and completely checked by a change 

 from 26 C. to 7 C. During the experiment take note 

 of any changes in the brightness of the sky ; if this 

 precaution is forgotten it is easy to be deceived by a 



1 Pringsheim's Jahrb. v. 



2 See also Pfeffer, Physiologic, i. p. 208. 



