and Laboratory Methods. 



1891 



II. RESPIRATION. 



1. To Demonstrate Respiration in Chlorophyll=bearing Plants. Ordinarily the 

 respiratory products are not demonstrable in the presence of those of photosyn- 

 thesis ; if the plant be deprived of light, however, the former are not obscured 

 by the latter. The usual method of confining a plant under a bell-jar and ex- 

 cluding light produces good results if the temperature is kept fairly constant in 

 order to avoid disturbances due to the expansion or contraction of air under the 

 bell-jar. The edge of the bell-jar should be ground and it should stand either on 

 a ground glass plate smeared with fat or in a shallow dish of mercury. After 

 numerous trials it has been found that a potted Coleus, on account of its exten- 

 sive leaf surface, gives better results than any other plant for this experiment. 

 After twenty-four hours the presence of carbon-dioxid may be detected by a 

 flame or by a saturated solution of barium hydroxid. 



2. To Determine the Composition of tlie Gas Evolved During Respiration. 

 The formation of carbon dioxid by respiring plants may be very successfully 



Fig. 2. — An apparatus for determining the presence of carbon dioxid in the gas 



evolved by plants. 



demonstrated in the laboratory or in the lecture room by conducting the gases 

 from the culture chamber through a saturated solution of barium hydroxid and 

 precipitating the carbon dioxid as barium carbonate. Set up the apparatus 

 shown in Fig. 2. Bottle C contains three or four seedlings and a few pieces of 

 moist blotting paper ; the four wash bottles are tilled with filtered barium hy- 

 droxid just before starting the experiment. Bottle F acts as an aspirator to draw 

 a current of air through the apparatus. A clamp attached to the siphon is so 

 Regulated that the water flows in drops. At the beginning of the experiment 

 bottle C should be connected directly with F and the aspiration current allowed 

 to run until the air in the first three bottles has been replaced by air containing 

 no carbon dioxid ; when a liter or more of water has run off the bottles may be 

 connected as shown in the figure. At the end of thirty minutes the solution in 

 D should contain a white precipitate of barium carbonate, proving the escape of 

 carbon dioxid from C. Bottles B and E are inserted to insure the removal of all 

 the carbon dioxid. 

 University of Michigan. Howard S. Reed. 



