AMOUNT OF WATER EXUDED AND PRESSURE OF EXUDATION 257 



electro-magnet together with a clock-work arrangement which produces electrical 

 contact at intervals. A photographic registration is also possible, and may be 

 advantageous, as also the use of a Brudon's spring manometer to register either 

 the positive exudation-pressure or the negative sucking force (Vines) '. 



SECTION 43. Amount of Water Exuded and the Pressure of Exudation. 



If saturated with water a plant begins to bleed the moment it is 

 decapitated ; otherwise the exudation of water commences after a longer 

 or shorter interval, and gradually rises to a maximum '. After one to two 

 days the flow from a stump connected with the roots begins to decrease, and 

 in many plants ceases after four to seven days. Others bleed for a longer 



-a 



FIG. 34. 



FIG. 35. 



time, however ; according to Hartig a woody plant may continue to exude 

 water for a month, and the decapitated flowering axis of Agave for as long 

 as five months (Humboldt) 3 . Independently of the fact that the roots 

 ultimately become sickly, the decrease of the flow is due to the vessels in the 

 neighbourhood of the wound becoming gradually blocked (Sect. 36), and 

 hence by exposing a fresh surface the escape of water may be caused to 

 recommence, as was first observed by Duhamel 4 . 



Cut shoots only begin to exude water after they have been immersed 



1 Baranetzky, Unters. liber d. Petiodicitat d. Blutens, 1873, pp. 19, 23 (Sep.-abdr. a. d. Abhand- 

 lungen d. Naturf.-Ges. zu Halle, Bd. Xlil). The apparatus given in Fig. 35 is prepared by the instru- 

 ment maker Albrecht, in Tubingen. On photographic registration, see Langendoiff, Physiol. Graphik, 

 1891, p. 90; Vines, Annals of Botany, 1896, Vol. x, p. 291. [The negative pressures observed by 

 Vines were due to physical imbibition on dead branches combined with evaporation.] 



2 Baranetzky, Unters. iiber d. I'eriodicitiit d. Blutens, Abhandlungen d. Naturf.-Ges. zu Halle, 

 1873, Bd. xiil, p. 30; Brosig, Die Lehre v. d. Wurzelkraft, 1876, p. 24. 



3 Quoted by Meyen, Pflanzenphysiol., 1838, Bd. n, p. 85 ; Th. Hartig, Bot. Zeitung, 1862, p. 89. 

 * Duhamel, Naturgesch. d. Banme, 1764, Bd. I, p. 89. 



PFEFFER S 



