TRANSPIRATION. 47 



immersed in water. The air is forced out through the stomata and rises as 

 bubbles to the surface of the water. A.t the close of the experiment some 

 of the air bubbles will still be in contact with the leaf surface at the opening 

 of the stomata. The pressure of the water gradually forces this back into 

 the leaf. Other plants will answer for the experiment, but some are more 

 suitable than others. 



92a. Number of stoiuata. The larger number of stomata are on the 

 under side of the leaf. (In leaves which float on the surface of the water 

 all of the stomata are on the upper side of the leaf, as in the water lily.) It 

 has been estimated by investigation that in general there are 40-300 stomata 

 to the square millimeter of surface. In some plants this number is exceeded, 

 as in the olive, where there are 625. In an entire leaf of Brassica rapa 

 there are about 11,000,000 stomata, and in an entire leaf of the sunflower 

 there are about 13,000,000 stomata. 



92b. Amount of water transpired by plants. The amount of water 

 transpired by plants is very great. According to careful estimates a sun- 

 flower 6 feet high transpires on the average about one quart per day; an 

 acre of cabbages 2,000,000 quarts in four months; an oak tree with 700,000 

 leaves transpires about 180 gallons of water per day. According to von Hoh- 

 nel, a beech tree no years old transpired about 2250 gallons of water in 

 one summer. A hectare of such trees (about 400 on 2^ acres) would at the 

 same rate transpire about 900,000 gallons, or about 30,000 barrels in one 

 summer. 



