38 PHYSIO LOG y. 



stake, in another jar covering no plant, another piece of cobalt 

 paper. They should both be put under the jars at the same 

 time. In a few moments the paper in the jar with the plant will 

 begin to redden. In a short while, ten or fifteen minutes, prob- 

 ably, it will be entirely red, while the paper under the other jar 

 will remain blue, or be only slightly reddened. The water vapor 

 passing off from the living plant comes in contact with the sensi- 



ig- 48. Fig. 49. 



Fig. 48. Water vapor is given off by the leaves when attached to the living plant- 

 It condenses into drops of water on the cool surface of the glass covering the plant 



tig. 49. A good way to show that the water passes off from the leaves in the form 

 of water vapor. 



tive cobalt chloride in the paper and reddens it before there is 

 sufficient vapor present to condense as a film of moisture on the 

 surface of the jar. 



74. Experiment to compare loss of water in a dry and a 

 humid atmosphere. We should now compare the escape of 

 water from the leaves of a plant covered by a bell jar, as in the 

 last experiment, with that which takes place when the plant is 



