162 



PRACTICAL COURSE IN BOTANY 



i8o. Magnitude of the work of transpiration. — Few 



people have any idea of the enormous quantities of water 

 given off by leaves. It has been calculated that a healthy 

 oak may have as many as 700,000 leaves, and that 111,225 

 kilograms of water — equal to about 244,700 pounds — may 

 pass from its surface in the five active months from June 



to October. At 

 this rate 226 

 times its own 

 weight may pass 

 through it in a 

 year, and it 

 would transpire 

 water enough 

 during that time 

 to cover the 

 ground shaded 

 by it to a depth 

 of 20 feetli 

 Lawn grass gives 

 off water at such 

 a rate that a va- 

 cant lot of 150 X 

 50 feet, if well 

 turfed, would be 

 capable of trans- 

 piring over a ton 

 of water a day. Compare these figures with the average yearly 

 rainfall in our Gulf States — 53 inches, approximately — and 

 you can form some estimate of the injury done to a growing 

 crop from this cause alone. The moisture is drawn from the 

 surface by shallow rooted weeds (81) and dissipated through 

 the leaves. In the case of forest trees the effect is different. 

 Their roots, striking deep into the soil, draw up water from 

 the lower strata and distribute it to the thirsty air in summer. 



Fig. 217. — A "weeping tree," showing the effect where 

 absorption exceeds transpiration. Notice the position of 

 the tree near the water where the roots have unlimited 

 moisture. {After France.) 



1 Marshall Ward, " The Oak.' 



