MOVEMENT OK MATERIALS IN THE IM. AN I 



C37 



The amount of water lost from plants by evaporation is very Large; in 

 Wiesner's experiments, for instance, three maize seedlings weighing 1.6 g. lost 

 0.198 g. of water during a single hour in sunlight. Wollny 1 measured the 

 amount of water lost by evaporation from several plants during their entire- 

 vegetative period and also determined the dry weights of the harvested plants 

 and the amounts of water evaporated for each gram of dry material for the 

 entire period of growth.' These values, in grams, appear in the table below. 



Although plants evaporate large amounts of water, as is evident from 

 the data just given, the amount of water lost from a certain area of leaf is con- 

 siderably less than that lost from an equal area of a free water surface. Ac- 

 cording to Hartig, 1 sq. m. of free water surface lost 2000 cc. of water in 

 twenty-four hours, while an equal area of beech leaves lost only 210 cc. y 



power of leaves, see: Livingston, В. E., The resistance offered by leaves to transpirational 

 water loss. Plant world 16 : 1-35. 1913. Bakke, A. L., Studies on the transpiring power of 

 plants as indicated by the method of standardized hygrometric paper. Jour. ecol. 2 : 145- 

 173. 1914. Livingston, В. E., and Shreve, Edith В., Improvements in the method for 

 determining the transpiring power of plant surfaces by hygrometric paper. Plant world 19 : 

 287-309. 1916. — Ed. 



1 Sachsse, Robert, Lehrbuch der Agriculturchemie. Leipzig, 1888. P. 423. [Whollny, E., Der Einflus s 

 der Pflanzendecke und der Beschattung auf die physikalischen Eigenschaften und die Fruchtbarkeit des 

 Bodens. 197 p. Berlin, 1877. P. 126.] 



i This ratio has been called the water requirement. For an excellent review of the literature 

 of this subject see: Briggs, L. J., and Shantz, H. L., The water requirement of plants. II. 1 A 

 review of the literature. U. S. Dept. Agric, Bur. Plant Ind., Bull. 285. 1913. — Ed. 



»' Such comparisons are without very much significance unless the two surfaces that are com- 

 pared have the same shape and the same exposure. In such studies as that here referred to it 

 has frequently been the practice to compare evaporation rates from circular, horizontally 

 exposed, free water surfaces with the corresponding rates of transpiration from an equivalent 

 area of plant leaves. The form and exposure of the latter surface is generally exceedingly com- 

 plex, while these characters of the water surface are relatively simple, and no very useful com- 

 parison is possible by such methods. The evaporating surface of the physical apparatus must 

 resemble the plant surface, in form, size, color, etc., as nearly as is practicable. In this connec- 

 tion, see Renner, О., Experimentelle Beiträge zud Kenntnis der Wasserbewegung. Flora 103 : 

 171-247. 1911. Idem, Zur Physik der Transpiration. Ber. Deutsch. Bot. Ges. 29 : 125-132. 

 1911. Idem, Zur Physik der Transpiration II. Ibid. 30: 572-575. 1912. Perhaps the 

 Livingston spherical porous-cup atmometer furnishes the best evaporating surface for com- 

 parison with plants in general, but for detailed study a special atmometer constructed after the 

 pattern of the particular plant used should be employed. On the porous-cup atmometer 

 sec: Livingston, В. E., Atmometry and the porous-cup atmometer. Plant world 18: 21-30, 

 51-74, 95-111, 143-149. 1915. Livingston, В. E. and Thone, Frank, A simple non-absorb- 

 ing mounting for porous porcelain atmometers. Science, n. s. 52: 85 86. 1020. — Ed. 



