AMOUNT OF WATER EXUDED AND PRESSURE OF EXUDATION 261 



connected with a glass tube filled with water whose position was similarly 

 changed l . Under certain conditions an injury may cause a marked fall in 

 the pressure registered by a manometer, as Horvath observed on cutting 

 off a lateral root of Helianthus anmius*. 



The amount of fluid that escapes is dependent upon the relation 

 between the resistance offered by the channels through which water is 

 driven, and the forces which push it onwards. It is therefore the resultant 

 of two varying factors, and hence a high exudation-pressure does not 

 necessarily produce a copious flow. The highest manorrietric pressure is 

 reached when the driving force is just counterbalanced, and a condition 

 of equilibrium attained. The exudation-pressure is only dependent within 

 certain limits upon the rapidity with which water is driven inwards and 

 onwards through the plant, for as the pressure increases, water is forced 

 into the vessels less and less rapidly, and hence, as Hales first showed, 

 the higher the column to be supported the more slowly the mercury rises 

 in the manometer. As the pressure upon the cut surface increases, the 

 amount of fluid escaping must therefore necessarily decrease 3 , provided that 

 the plant calls no compensating regulatory mechanism into play. The 

 apparatus given in Fig. 33 may be used for an experiment of this kind, 

 by making the open arm of the manometer of the required height and 

 collecting the mercury as it flows over. If the manometer bulb (;) is 

 sufficiently large, it takes some time before the water exuded from the 

 stem begins to interfere with the action of the instrument. Detmer has 

 shown that the rate of flow increases when the atmospheric pressure 

 diminishes 4 . 



Since pieces of the stem are capable of originating active exudation, 

 it is impossible to predict how the escape of sap will be influenced by 

 the length of stem left attached to the roots. Detmer found in Ricinus 

 no difference in the amount of sap excreted from plants decapitated near 

 the base and others decapitated higher up, but Baranetzky, in his ex- 

 periments with Ricinus and Helianthus annutts*, observed that the flow 

 was more conspicuous from the plant with the longer piece of stem. No 

 conclusions can be drawn from the unequal amounts of fluid that have 

 been observed to escape through manometers placed at different levels 6 , 



1 See Hofmeister, Flora, 1862, p. 117. 



2 Similar observations are given by Th. Hartig, Bot. Zeitung, 1863, p. 281. 



3 As Sachs found (Lehrb., 1874, 4. Aufl., p. 658). [According to Chamberlain (Bull. d. Lab. d. 

 Geneve, 1897, T. II, p. 330), the plants respond by accelerating the flow. Chamberlain has not, 

 however, kept the direct action and the physiological reactions clear and distinct from one another.] 



4 Detmer, in Schenk u. Liirsscn, Mitth. a. d. Gesammtgeb. d. Bot., 1874, pp. 439, 453; 

 Wieler, Cohn's Beitrage, 1893, Bd. vi, p. i 7 



5 Detmer, Beitrage z. Theorie d. Wurzeldruckes, 1877, P- a8 > Baranetzky, Abh. d. Naturf.- 

 Ges. z. Halle, 1873, Bd. xm, p. 52. 



8 Cf. Unger, Sitzungsb. d. Wien. Akad., 1857, Bd. xxv, p. 447. 



