204 MOVEMENTS AND EXCHANGES OF FLUIDS 



tions, among which the stage of development and differentiation 

 of the tissues may be mentioned. 1 



275. Amount of Transpiration. Haberlandt found that a single 

 individual of maize transpired 14 kg. of water during its develop- 

 ment, which occupied 173 days, an individual of hemp 27 kg. in 

 its development in 140 days, and a single plant of the sunflower 

 the same amount during its development lasting the same length 

 of time. Hales found that a sunflower having a total leaf surface 

 of 9 sq. m. transpired .85 kg. in a single day. The amount of 

 water transpired by a plant in any given period is generally about 

 equal to the amount absorbed, provided the substratum furnishes 

 a suitable supply of moisture. Two general methods may be 

 used to estimate transpiration. By one method the plant is pro- 

 vided with a supply of water, and set on a balance so that the 

 amount of water lost, can be detected by the loss in weight. The 

 second method entails the measurement of the amount of water 

 taken up under conditions, which must be uniform for some time 

 before the beginning of the test. Still a third method of limited 

 usefulness consists in enclosing the plant in a closed vessel con- 

 taining a known weight of some substance that will absorb watery 

 vapor from the air. The increase in the weight of the absorb- 

 ent denotes the amount of transpiration. 2 



276. Determination of Transpiration by Weighing. Provide a 

 plant with large leaf surface growing in a 3 -inch pot and set it in 

 a tin pail slightly, larger and deeper, and containing a few cc. 

 of water to keep the soil moist. Tie a piece of oiled cloth over 

 the top of the pail and tightly around the stem to prevent the 

 escape of watery vapor. Bring the edges of the cloth down 

 under the pail and fasten. The whole preparation should not 

 exceed 800 g. in weight. Weigh exactly on a balance of suffi- 



1 For complete bibliography, see Burgerstein. Materialen zu einer Monographic d. 

 Transpiration d. Pflanzen. I. 1887. II. 1889. III. 1900. 



2 Kohl. Transpiration. 1 886. 



Pfeffer, W. Physiology of Plants, 228. 190x5. 



Copeland, E. B. A new self-registering transpiration machine. Bot. Gazette. 

 26: 243. 1898. 



