242 



THE MOVEMENTS OF WATER 



absorption clearly visible. Since the tube (a) is horizontal, no alteration in the water- 

 pressure is produced as transpiration continues, while by opening a tap or clamp 

 at b fresh water can be allowed to flow into a and fill it when nearly emptied . 



When a transpiring plant is covered by a bell-jar, the dew that condenses upon 

 the inner surface of the latter affords direct evidence that the plant is exhaling 

 water-vapour. If a vessel containing anhydrous calcium chloride is placed under 

 the bell-jar, the increase in weight of the deliquescent substance forms a measure 

 of the transpiratory activity of the objects employed, such as single leaves or even 

 different surfaces of the same leaf. The cobalt-paper method first employed by 

 Stahl is of great use for such comparative determinations 2 . 



For a description of the automatically registering apparatus used by Vesque, 



Eder, Krutizsky, Marey, and 

 Anderson, the reader is re- 

 ferred to their original works 3 . 

 It is not difficult to cause a 

 beam- or, better, a spring- 

 balance to register automati- 

 cally any change in weight, 

 while by means of a float 

 used in conjunction with a 

 simple mechanical arrange- 

 ment the absorption of water 

 in a potometer may be 

 graphically registered *. 



The surface area of a leaf 

 is most easily determined by 

 preparing a piece of paper of 

 corresponding outline, and 

 finding the weight of a unit of 

 area of the latter. The outline of the leaf may be traced upon the paper in pencil, or 

 obtained photographically by using ferrous or albumenized paper. The use of a plani- 

 meter is hardly necessary, while the determination of the area by means of a glass plate 

 divided into squares is neither so accurate or convenient as is the method of weighing" 1 . 



FIG. 30. 



1 Potometers of this kind were used by Vesqne, Ann. d. sci. nat, 1868, vi. se'r., T. vi, p. 183 ; 

 Moll, Archiv. Neerlandaises, 1884, T. XVIII ; Bonnier et Mangin, Ann. d. sci. nat., 1884, vi. se>., 

 T. xvn, p. 288 ; Kohl, Transpiration, 1886, p. 61 ; Eberdt, ibid., 1889, p. 8. 



a Stahl, Bot. Zeitung, 1894, p. 118. Merget (Compt. rend., 1878, T. LXXVlli, p. 293) used 

 a similar paper prepared with chlorides of iron and palladium. 



8 Vesque, Ann. d. sci. nat., 1868, vi. se"r., T. vi, p. 186; Eder, Unters. iiber d. Aussch. von 

 Wasserdampf, 1875, p. 106; Sep.-abdr. aus Sitzungsb. d. Wien. Akad., Bd. LXXII, Abth. i; 

 Krutizsky, Bot. Zeitung, 1878, p. 161 ; Marey, Me"thode graphique, 1878, p. 276; Anderson, 

 Minnesota Bot. Studies, 1894, p. 177. 



* See Langendorff, Physiol. Graphik, 1891. Several forms of apparatus for registering growth 

 will be described later. 



J 1 The former method was used by Haberlandt (Wissensch. prakt. Unters. a. d. Gebiete d. 

 Pflanzenbaues, 1877, B ^. n, p. 140), the latter by Hales (Statics, 1748, p. 2) and Unger (Sitzungsb. 

 d. Wien. Akad., 1861, Bd. XLII, p. 195). 



