24 EXPERIMENTAL PLANT PHYSIOLOGY 



Inferences. — The water that has appeared in the 

 upper cylinder must have been given off from the leafy 

 branch in the form of vapour and then have condensed 

 on the sides of the glass. Thus : The leaves of a plant 

 give off water-vapour. This process is termed trans- 

 piration. 



If the above experiment be kept under observation a 

 little longer it is observed that the amount of water on 

 the upper cylinder does not go on increasing. 



This result seems at first a little contrary to expecta- 

 tion. It will be realised, however, by this time, that 

 no experiment can give a false result; whatever the 

 result, it must be true for the conditions set up. 



The giving off of water from the leaves into the air 

 is somewhat analogous to the drying of clothes on a 

 clothes-line. The drier the air is the more quickly do 

 the clothes give up the water contained in them. If 

 this is also the case with the leaves it is seen at once 

 that they have been put into an atmosphere which 

 very soon becomes charged with the moisture that they 

 themselves are giving off, and so the process under 

 observation is checked and finally ceases entirely. 



An experiment can now be set up to find out whether 

 the rate at which water is given off by the plant really 



A is affected by the dryness 



es or dampness of the air 



/C— ^ surrounding it. 



Experiment 12 



Aim. — To find out 

 _ whether the rate of trans- 

 piration is affected by the 

 dampness or dryness of the 

 surrounding air. 

 Fig. 10 M e t h d. — C a 1 c i u m 



chloride must be used in 

 this case. This substance has the power of absorbing 

 moisture from the air around it. 



