TRANSPIRATION AND EVAPORATION COMPARED. 275 



mentioned, and is of easy application. Two tubulated bell-jars, 

 each furnished with a mercury trap (m and in,'}, are secured firmly 

 with soft wax to opposite sides of any large leaf. In each bell- 

 jar is a small capsule (c and c') containing dry calcic chloride of 

 known weight. After a given time the salt placed in each bell- 

 jar is weighed, and the excess over its original weight shows the 

 amount of water transpired. The following are some of Gar- 

 reau's results : - 



(1) The quantity of water exhaled by the upper face of a 

 leaf is to that exhaled by the lower as 1:1, 1:3, or some- 

 times as 1:5. 



(2) There are marked but not exact relations between the 

 quantity of water exhaled and the number of stomata. 1 



738. Transpiration compared with evaporation proper. The 

 evaporation from a given surface of water is between three and 

 six times as great as that from an equal surface of green leaves 

 similarly exposed. Unger 2 found that leaves of Digitalis pur- 

 purea with a surface of live thousand square millimeters tran- 

 spired from 3.232 to 1.232 grams in a given time ; while from an 

 equal surface of water from 4.532 to 8.459 grams evaporated. 

 Sachs 3 found that from a surface of sunflower stem and leaf meas- 

 uring 4,920 centimeters enough water transpired to form a layer 

 2.23 mm. thick over the same surface ; while from an equal sur- 

 face of water enough evaporated to lower the level 5.3 mm. 



Sachs also found that the evaporation from an animal mem- 

 brane is greater than that from an equal surface of free water. 

 When a surface of water is covered by a moist layer of vegetable 

 parchment, evaporation is somewhat retarded ; 4 but even then it 

 is greater than that from an equal surface of leaves. 



But the area of a leaf does not express its evaporating sur- 

 face, since the latter consists of intercellular spaces which have 

 been estimated to bear the ratio of ten to one to the cuticularized 

 exterior. In the intercellular spaces the air is saturated with 

 moisture, hence the slowness of the rate of transpiration. 5 



739. Effect of moisture in the air upon transpiration. All ex- 

 periments show that with increase in the amount of aqueous 

 vapor contained in the air the amount of water transpired from 



1 Ann. des. Sc. nat., ser. 3, tome xiii., 1849, p. 321. Bonnet's early ex- 

 ptiments are interesting. 



2 Sitzungsb. d. Wiener Akad., Bd. xliv., Abth. 2, 1861, p. 206. 



3 Handbuch der Experimental-physiologic, 1865, p. 231. 



4 Baranetzky : Botanische Zeitnng, 1872. p. 65. 

 6 American Naturalist, 1881, p. 385. 



