162 PRACTICAL COURSE IN BOTANY 
180. 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 feet!! 
Lawn grass gives 
off water at such 
a rate that a va- 
cant lot of 150 x 
50 feet, if well 
Fic. 217.— A “ weeping tree,’’ showing the effect where 
absorption exceeds transpiration. Notice the position of turfed, would be 
the tree near the water where the roots have unlimited capable of trans- 
moisture. (After FRANCE.) piring over a ton 
of watera 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. 
' Marshall Ward, ‘‘ The Oak.” 
