34 PHYSIOLOGY. 



we infer that the water escapes from the leaves in the form of 

 water vapor, and that this water vapor, when it comes in contact 

 with the surface of the cold glass, condenses and forms the mois- 

 ture film, and later the drops of water. The leaves of these cut 

 shoots therefore lose water in the form of water vapor, and thus 

 a loss of turgidity results. 



72. Loss of water from growing plants. ^Suppose we now 

 take a small and actively growing plant in a pot, and cover the 

 pot and the soil with a sheet of rubber cloth which fits tightly 

 around the stem of the plant (or the pot and soil may be enclosed 

 in a hermetically sealed vessel) so that the moisture from the soil 

 cannot escape. Then place a bell jar over the plant, and set in 

 a brightly lighted place, at a temperature suitable for growth. 

 In the course of a few minutes on a dry day a moisture film forms 

 on the inner surface of the glass, just as it did in the case of the 

 glass jar containing the cut shoots and leaves. Later the mois- 

 ture has condensed so that it is in the form of drops. If we have 

 the same leaf surface here as we had with the cut shoots, we shall 

 probably find that a larger amount of water accumulates oti the 

 surface of the jar from the plant that is still attached to its 

 roots. 



73. Water escapes from the surfaces of living leaves in the 

 form of water vapor. — This living plant then has lost water, 

 which also escapes in the form of water vapor. Since here there 

 are no cut places on the shoots or leaves, we infer that t4ie loss 

 of water vapor takes place -from the surfaces of the leaves and 

 from the shoots. It is also to be noted that, while this plant is 

 losing water from the surfaces of the leaves, it does not wilt or 

 lose its turgidity. The roots by their activity and pressure sup- 

 ply water to take the place of that which is given off in the form 

 of water vapor. This loss of water in the form of water vapor by 

 plants is transpiraiion. 



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 



