THE MOVEMENTS OF WATER 



and no flagging occurs. Similarly, if a fresh cut is made under water 

 5 to 6 cm. above the one injected with air, the new surface absorbs water 

 at a sufficient rate to replace what is being lost by transpiration. The 

 same result may be produced by forcing water under pressure into the 

 stem. Thus the pressure of a column of mercury 20-40 cm. high generally 

 suffices to make flaccid shoots turgid again (Fig. 27), and also restores to 

 them the power of absorbing water against negative pressure. If the 

 experiment is performed with the apparatus figured on p. 211 (Fig. 26) 

 the mercury will be raised in the arm a and supported there 1 . 



FlG. 27. The flaccid shoot of Intpatiens parvi- 

 Jlora has passed in one hour from the position (a) 

 to that of (6), owing to the pressure of the mercury 

 forcing water into it. 



FlG 



To show the relationship between transpiration and the absorption of water the 

 simple apparatus depicted in Fig. 28 may be employed. A cut stem or young 

 rooted plant is fixed so that the root system or lower end of the stem is immersed 

 in the water which fills the air-tight cylinder g. The amount of water absorbed 

 is registered by the scale on the tube n, and by placing the entire apparatus on 

 a balance the weight lost by transpiration can be determined. A somewhat 

 different arrangement may be used for more exact experiments (cf. Sect. 38). 

 By such means it has been shown from the time of Hales onwards that under 

 constant conditions the amounts of water absorbed and exhaled correspond *, and 

 that any change of conditions tending to produce an increased rate of transpiration 



1 After Sachs (1870) had called attention to this phenomenon, de Vries studied it more closely 

 (Arb. d. Bot. Inst. in Wurzburg, 1873, Bd. i, p. 287). The explanation given above is due to 

 v. Hohnel (Haberlandt's wiss. prakt. Unters., 1877, Bd. II, p. 129). See also Strasburger, Bau u. 

 Verricht. d. Leitungsbahnen, 1891, p. 680. 



2 Hales, Statics, 1748, p. 18. Later, Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLiv, p. 360 ; 



