CH. II] EXCESS OF CO 2 . 27 



kept a long time in the dark before they are destarched. 

 Two graduated jars of 200 c.c. capacity are filled with 

 and inverted over water, and plants of Gallitriche, which 

 have been previously deprived of starch, are passed under 

 the edge and allowed to float up. Into one jar equal 

 quantities of air and CO 2 , while into the other 12 

 volumes of air to one of C0 2 are passed. The propor- 

 tion of CO 2 in the atmospheres so prepared does not of 

 course remain constant, since the water absorbs the gas. 

 But if the experiment is started in the evening and 

 concluded in the evening of the next day, one jar will 

 certainly contain far more than the optimum of C0 2 , 

 while the other will not fall much below the optimum. 

 A still simpler plan is to use beakers of about 800 c.c. 

 capacity inverted in saucers of water. The beakers are 

 graduated as follows : into one 550 c.c. of water is poured 

 and the level marked with a diamond, a second mark 

 being made after the addition of 50 c.c. The other beaker 

 is marked at 300 and 600 c.c. The beakers are filled 

 with water and inverted in saucers, and the rosettes of 

 Callitriche floated up under the rims of the beaker. 

 Three hundred c.c. of air are now introduced into one 

 beaker and 550 c.c. into the other, using a finger bellows 

 for the purpose; afterwards CO 2 is added until each beaker 

 contains 600 c.c. of mixed gas, one containing 50 p.c., the 

 other 8 p.c. of C0 2 . In our experiments the Callitriche 

 exposed to 50 c.c. C0 2 showed hardly any starch, while the 

 control-plants were black with it. 



The experiment may be more accurately performed 

 with a pair of graduated tubes inverted over mercury 



