72 EXPERIMENTAL PLANT PHYSIOLOGY 



A control experiment, in which the potted plant is 

 omitted, is set up. 



After an hour or two the lime-water in the two 

 dishes is compared. 



Observations. — 1. The lime-water in the bottle which 

 contains the peas becomes milky. That in the control 

 experiment remains unchanged. 



2. The taper is immediately extinguished. 



3. Milkiness is found only in the dish of lime-water 

 that was under the bell-jar that covered the plant. 



Inference.^ — A 'plant gives out carbon-dioxide; this, 

 then, may be an indication that plants, like animals, 

 breathe. 



It will be remembered that the air is a mixture of 

 oxygen and nitrogen together with a small amount of 

 carbon-dioxide, water-vapour, and various rare gases 

 and impurities in small quantities. 



Oxygen is a very active gas ; a glowing splint, when 

 thrust into a jar of this gas, bursts into flame ; a burn- 

 ing taper burns more brightly in oxygen than in air. 

 Nitrogen, on the other hand, is inert. It will, therefore, 

 be readily understood that, if air is taken in by the 

 plant for respiration, it is the behaviour of the oxygen 

 that must be considered. An experiment will now be 

 made to ascertain whether the plant retains oxygen in 

 exchange for the carbon-dioxide given out. 



Experiment 43 



Aim. — To find out whether a plant takes in oxygen 

 from the air. 



Method. — Damp blotting-paper, germinating peas, 

 and a test-tube of potash solution are put into a glass 

 flask. Through the cork of the flask is fitted a piece 

 of narrow glass-tubing bent twice at right angles as 

 shown in Fig. 35, and having its free end dipping into 

 a dish of coloured liquid. 



If this experiment is to succeed the cork must be 



