20 THE LEAF 



20. Practical Effects of Transpiration. — Where does all 

 this moisture come from ? If the water in the last experi- 

 ment is colored with a little eosin or with red ink, its 

 course can be traced through the stem into the leaves. In 

 growing plants the earth takes the place of our tumbler of 

 water, and from it the moisture is drawn up by the roots 

 and conveyed through the stem to the leaves. Thus we see 

 that trees are constantly acting as great pumps, drawing 

 up water from the lower strata of the soil and distributing 

 it to the thirsty air in summer. As the water given off 

 by transpiration is in the form of vapor, it must draw from 

 the plant the amount of heat necessary for its vaporization, 

 and hence it has the effect of making the leaves and the air 

 in contact with them cooler than the surrounding medium. 



21. The Cause of Transpiration. — The reason why 

 plants exhale such large quantities of water is because they 

 get part of their food from mineral and other substances 

 dissolved in the water of the soil, but this food is in such a 

 diluted state that enormous quantities of the liquid contain- 

 ing it must be taken up in order to give the plant the nour- 

 ishment it requires. This liquid travels through the stem 

 as sap, and after all the food substance has been extracted, 

 the waste water is exhaled by the leaves. Sometimes the 

 roots absorb moisture faster than the leaves can transpire 

 .it ; the water then exudes through the stomata and settles 

 in drops on the blade, causing the leaves to sweat, just 

 as our bodies do under similar conditions. Sometimes, on 

 the other hand, the leaves transpire faster than the roots 

 can absorb, and then the plant wilts. 



PRACTICAL QUESTIONS 



1 . Do you see any connection between the facts just stated and the 

 stories of " weeping trees " and " rain trees " that we sometimes read 

 about in the papers ? (Section 21.) 



2. Can you explain the fact sometimes noticed by farmers, that in 

 wooded districts, springs which have failed or run low during a dry 

 spell sometimes begin to flow again in autumn when the trees drop 

 their leaves, even though there has been no rain? (19, 20.) 



