74 EXPERIMENTAL PLANT PHYSIOLOGY 



oxygen inspired (i.e. breathed in) in any given time. 

 The original and final levels of the liquid must be 

 marked with gummed paper. The amount of space 

 between the two marks can be calculated from the 

 formula tttH, where 7r=\^-, 7' = the radius of the tube, 

 and l = the length between the two marks. 



This gives only a rough approximation of the quantity 

 of oxygen taken in, since other factors, inseparable with 

 the experiment, make the amount of oxygen absorbed 

 appear less than it really is. Of these factors two very 

 important ones are the evaporation of water-vapour 

 from- the peas and the blotting-paper and the rise in 

 the temperature of the air withm the flask (see Experi- 

 ment 49) ; both these factors are forces producing a 

 downward pressure on the surface of the liquid in the 

 tube. 



The error due to rise of temperature may be elimi- 

 nated if a thermometer is fitted into Ihe cork and the 

 temperature kept constant by surrounding the flask 

 with a wet cloth. 



Experiment 45 



Aim. — To find out the relation between the volume 

 of oxygen taken in by the plant 

 and that of carbon-dioxide given 

 out. 



M e t h d. — Damp blotting- 

 paper and germinating peas are 

 put into a flask, but the test- 

 tube of potash solution is in this 

 case omitted. A long piece of 

 glass -tubing of a fairly wide 

 bore is bent as in Fig. 36 and 

 fitted into the cork or, better, 

 into a rubber-stopper. A ther- 

 mometer is also put through the 

 cork. 



Some coloured liquid is next 

 poured into the glass tube ; the liquid should be suffi- 



