and Laboratory Methods. 



1927 



METHODS IN PLANT PHYSIOLOGY. 



III. 



RESPIRATION— Continued. 



3. The Respiratory Ratio in Starchy and Oily Seeds. The ratio of the vol- 

 ume of oxygen inhaled to the volume of the carbon dioxid exhaled is usually 



CO 

 expressed by the formula — ~^=1, which means that the volume of the oxygen 



absorbed is equal to the volume of the carbon dioxid given off. This ratio prob- 

 ably holds true where the oxygen is used only for respiration, but in the germi- 

 nation of oily seeds part of the oxygen is used to 

 convert the oil into starch, thus causing the ab- 

 sorption of oxygen to be greater, volumetrically, 

 than the evolution of carbon dioxid. The differ- 

 ence may be demonstrated in the following man- 

 ner : Place about 20 g. of imbibed seeds of wheat 

 or barley in a 500 c. c. Erlenmeyer flask, con- 

 nect it with a vessel of mercury by means of a 

 continuous piece of glass tubing ; prepare a pre- 

 cisely similar piece of apparatus, using seeds 

 of hemp or flax. A thermometer is inserted in 

 the rubber stopper in the mouth of the flask 

 and all joints tightly sealed (Fig. 3). By gently 

 warming the flask a little air is driven out and 

 mercury is drawn into the glass tube ; in this 

 way the lower portion of the tube is filled with 

 mercury. When the flask has cooled to room 

 temperature mark the position of the mercury 

 with a thread or strip of gummed paper, at the 

 same time recording the temperature of the 

 flask. Keep the apparatus from direct sunlight 

 in a room of even temperature. The final ob- 

 servation should be made eighteen to twenty 

 hours later ; at that time bring the flask to the 

 same temperature as at the beginning of the 

 experiment, and observe the point to which the mercury column extends. The 

 starchy seeds will cause an increase in the volume of gas, while the oily seeds 

 will show a decrease in the volume of the gas in the flask. 



4. Intramolecular Respiration. The evolution of carbon dioxid from plants 

 deprived of oxygen may be demonstrated successfully by either of the following 

 methods : 



(a.) Fill a test-tube with mercury, cover the mouth of the tube with a small 

 disk of cork and invert the tube over a dish of mercury. The disk of cork can 

 be easily slipped off, and holds the mercury better than the finger. Remove the 



Fig. 8. A means of demonstrating 

 the respiratory ratio. The hori- 

 zontal tube is about 60 cm. long 

 and terminates in a dish of mer- 

 cury. 



