EXCRETION OF CARBON DIOXIDE 255 



the rise of mercury in the tube. Note the amount of mercury 

 drawn up 4 and 8 hours later. This experiment is of value only 

 with plants in which the amounts of carbon dioxide exhaled and 

 oxygen absorbed are equal. It is subject to the following errors : 

 The proportion of oxygen in the air about the seeds is constantly 

 decreasing, which would lessen respiratipn, and the downward 

 pull of the column of mercury varying with the barometric pres- 

 sure would also tend to make the result less than the normal. 

 The actual volume of carbon dioxide given off will also be greater 

 than that of the mercuiy drawn up in the tube. 



This demonstration may also be accomplished in the follow- 

 ing manner : secure a funnel tube with a cylindrical top with a 

 capacity of 100 to 200 cc, and place 20 or 30 germinating seeds 

 on a piece of filter paper in the cylinder. Bend a section of 

 wire a few cm. in length to form a tripod support for a smajl 

 bottle containing a solution of sodium or potassium hydrate. 

 Close the upper end of the cylinder tightly with a large rubber 

 stopper. Now support the funnel tube in a perpendicular posi- 

 tion in a small dish of mercury. Warm the tube until some air 

 is driven out allowing the mercury to rise to the same level in 

 the tube and dish when cool. The -volume of mercury drawn up 

 in the tube represents amount of carbon dioxide absorbed, with 

 corrections as above (Fig. 126). 



322. Incomplete Combustion in Oily Seeds. A smaller propor- 

 tion of carbon dioxide is given off in the respiration of oily, than 

 starchy seeds. Soak a few dozen seeds of hemp for an hour, and 

 then lay on moist filter paper in the flask used in the last experi- 

 ment and set up as before, omitting the potassium. Note the 

 slight rise of the mercury due to the fact that in the earlier stages 

 of germination of such oily seeds, less carbon dioxide is excreted 

 than oxygen is absorbed. Later however, the formation of 

 starch restores the normal ration in the interchange of the two 

 gases, and the rise of the column of mercury will soon cease. 



323. Excretion of Carbon Dioxide in the Anaerobic Respiration 

 of an Aerobe. Pisum is an aerobic species but the seeds are capa- 



