INFLUENCE OF SPECIFIC PECULIARITIES ON TRANSPIRATION 241 



intercellular system is barely able to raise the evaporating power of a leaf 

 to the same level as that of a free surface of water. 



Methods. To demonstrate transpiration, we may employ the cobalt reaction 1 , 

 or any other hygrometric test. Quantitative estimations may be made either by deter- 

 mining the loss of weight, or by observing the amount of water absorbed. The 

 latter is an indirect method, and is only approximately correct at any given moment. 

 Woodward determined the transpiration by weighing the plant from time to time ; 

 Mariotte collected the exhaled water vapour as did Guettard, while Hales measured 

 also the amount of water absorbed -. These three methods have undergone various 

 minor modifications in the hands of later investigators. 



The balance shown in Fig. 29 may be used to measure the weight of water lost 

 by transpiration 3 . When 

 a plant in a pot is used, the 

 pot must be placed in a 

 vessel of lead or glass with 

 a perforated cover, through 

 which the stem may project 

 into the air 4 . When a 

 plant growing in water is 

 employed (Fig. 28, p. 232), 

 it must be fixed in the glass 

 vessel by means of an air- 

 tight cork, or the water may 

 be covered with a layer of 

 oil. This latter method 

 was used by Unger, but can 

 hardly be recommended 5 . 



If the apparatus by 

 which the amount of water 

 absorbed is measured, be 

 weighed from time to time, 

 it is found that under 

 normal conditions the absorption of water affords a sufficiently accurate indication 

 of the amount transpired. The apparatus shown in Fig. 30 is adapted for more 

 delicate measurements, since the narrow tube (a] renders a slight amount of 



1 [Paper impregnated with cobalt chloride is blue when dry but turns rose-pink as it absorbs 

 water, various shades of colouration indicating the relative degree of saturation.] 



- Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 206; Sachs, Experimentalphysiol., 1865, 

 p. 231 ; Woodward, Phil. Trans., 1699, Vol. xxi, Nr. 253, p. 198; Mariotte, CEuvres de Mariotte, 

 1717, p. 135 ; Guettard, Hist. d. 1'Acad. roy. de Paris, 1748, p. 571. 



3 Extremely good balances of this kind, weighing up to 5 kilos, may be obtained from Kern u. 

 Sohn in Ebingen, Wiirttemberg. 



4 Cf. v. Hohnel, Transp.-grosse forstl. Gewiichse, 1879, p. 4. 



5 Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 362. The changes in weight caused by 

 respiration, assimilation, or by the excretion of ethereal oils or other substances, are so trifling as to be 

 negligible in comparison with the loss in weight due to transpiration. The transpired water is found 

 to be nearly pure when collected. Cf. the older literature given by Treviranus, Physiol., Bd. I, p. 493. 



PFEFFER R 



FIG. 29. 



