22 A TEXT-BOOK OF BOTANY 



15. Transpiration. Water is being evaporated con- 

 stantly from the surface of a living plant exposed to the air. 

 This loss of water by the plant has been called transpiration. 

 Since leaves are especially exposed to the air, their transpira- 

 tion is conspicuous. Although the epidermis impedes trans- 

 piration, we have seen (13) that the leaf has in its system of 

 air-spaces an internal atmosphere, which is in communica- 

 tion with the external atmosphere through the stomata. 

 Hence, water vapor is constantly passing from the working 

 cells into the internal atmosphere and diffusing through the 

 stomata into the external atmosphere. Although a certain 

 amount of transpiration takes place directly through the epi- 

 dermal cells, much the larger part of the water vapor passes 

 out by way of the stomata. If the stomata are closed by the 

 guard-cells, the internal atmosphere becomes saturated with 

 water vapor and transpiration ceases. It is evident that 

 the larger the air-spaces in the leaf, that is, the looser the 

 leaf is in texture, the greater is the amount of internal 

 atmosphere, and the more rapid is transpiration. Hence 

 the amount of transpiration from a leaf depends more upon 

 its structure than upon the extent of its exposed surface. 



If a glass vessel (bell jar) be placed over a small active 

 plant, the moisture is seen to condense on the glass, and 

 even to trickle down the sides (Fig. 17).* When the 



as follows : After the exposure to light, the leaves were placed in alcohol 

 to extract the green coloring matter {chlorophyll}. When this was 

 accomplished, they were rinsed thoroughly, to remove the alcohol, and 

 placed in a water solution of iodine. In this solution the starch-con- 

 taining portion becomes dark blue, the other portion remaining colorless. 

 The water solution of iodine is obtained by dissolving potassium iodide 

 in water and adding scales of iodine. 



* Some such experiment should be performed to demonstrate the 

 fact of transpiration. Care must be taken to shut off the evaporation 

 from the pot or soil, since it is to be demonstrated that water is coming 

 from the plant. Rubber cloth or a coating of paraffin or wax may be 

 used for sealing up all sources of moisture except the plant (Fig. 17). 



