29 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. Ifthe 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 strueture 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). 
