92 GROWTH AND WORK OF PLANTS 





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.* 



155. 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. 74) 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 -f 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 favorable 

 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. 



156. Conditions which favor or retard transpiration. 

 Dry air favors transpiration, since it permits a more rapid diffusion 

 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 transpiration by the 

 leaves. Light also favors transpiration. Since osmotic substances 

 are more active in the guard cells, they become turgid, curve back- 

 ward and keep the stomates open. Humid air, darkness, or weak 



* 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. 



f 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 condensa- 

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



