WORK OF THE LEAVES: TRANSPIRATION 99 



has been given off to show on the glass jar. This demonstrates 

 that the water escapes from the leaves in the form of water vapor. 

 When a sufficient amount of water vapor is given off by the leaves 

 under the jar it condenses on the cool glass in the form of water.* 



159. How the water vapor escapes from the leaf. The 

 living cells of the leaf contain a large percentage of water in the 

 cell sap. The cell walls are saturated with water because they 

 imbibe or absorb water from the cells which they bound. In the 

 loose parenchyma of the leaf (see fig. 75) there are numerous 

 large intercellular spaces containing air. The water on the sur- 

 face of these cell walls is in contact with the air. This water 

 evaporates-)- into the air spaces, or passes off into water vapor. 

 The water vapor diffuses in the intercellular spaces, so that the 

 air becomes very humid, more so than the air outside of the leaf 

 on ordinarily dry days. The water vapor diffuses out of the leaf 

 through the open stomates, and makes room for more water to 

 evaporate from the cell wall. The cell wall in turn takes by imbi- 

 bition more water from the cell, so that under conditions favor- 

 able for this process water vapor is constantly flowing out through 

 the stomates. Some water also evaporates from the external 

 walls of the epidermal cells, but the quantity is usually small 

 because of the waxy cuticle over the epidermis. 



160. Conditions which favor or retard transpiration. 

 Dry air favors transpiration, since it permits a more rapid diffu- 

 sion of the water vapor out of the intercellular spaces. Currents 

 of air also hasten transpiration, since the water vapor is quickly 

 carried away from the surface of the leaf. This is why dry winds 

 or high winds often cause plants to wilt, especially when the 

 soil is dry and absorption by the roots is not equal to the trans- 

 piration by the leaves. Light also favors transpiration. Since 



* A good illustration of the condensation of water vapor on the surface 

 of a cool object is seen in the summer when the air is very humid and drops 

 of water accumulate on the outside of a pitcher of cold water. 



j" If it is desired to demonstrate the evaporation of water a shallow vessel 

 of water can be covered with a bell jar or tumbler. The condensation of 

 water on the inside shows some of the water has evaporated. 



