THE LEAF 163 



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 thus has the effect of 

 making the leaves and the air in contact with them cooler 

 than the surrounding medium. At the same time the cool- 

 ness and moisture of the air tend to check the loss by 

 evaporation from the surface soil. It is partly to this cause, 

 and not alone to their shade, that the coolness of forests is 

 due. Measurements at various weather bureau stations in 

 the United States show that in summer the temperature of 

 oak woods is 4° C. lower during the day than in the open, 

 and as much higher at night. In a beech wood in Germany 

 the difference between the forest and the general tempera- 

 ture amounted to as much as 7° C. 



Practical Questions 



1. Is there any foundation in fact for the accounts of "weeping trees" 

 and "rain trees" that we sometimes read about in the papers? (180; 

 Exp. 48.) 



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? (180; Exp. 63.) 



3. Other things being equal, which would have the cooler, pleasanter 

 atmosphere in summer, a well-wooded region or a treeless one? (180.) 



4. Could you keep a bouquet fresh by giving it plenty of fresh air? 

 (Exp. 62.) 



5. Why does a withered leaf become soft and flabby, and a dried one 

 hard and brittle? (7; Exp. 62.) 



6. Why do large-leaved plants, as a general thing, wither more quickly 

 than those with small leaves? (Exp. 63.) 



7. Is the amount of water absorbed always a correct indication of the 

 amount transpired ? Explain. (179.) 



8. Explain the difference between the withering caused by excessive 

 transpiration and the shrinkage of cells due to plasmolysis. Are both of 

 these physiological processes ? 



9. Why is it best to trim a tree close when it is transplanted ? (179, 

 180.) 



10. Why should transplanting be done in winter or very early spring, 

 before the leaves appear ? (180.) 



