258 THE MOVEMENTS OF WATER 



in water for some time, and often only after one or two days (Pitra, Wieler). 

 This is probably due to the feeble absorptive powers of the leaves and 

 branches, for when young branches are used, the bleeding generally com- 

 mences sooner, and it may even appear almost immediately if they are 

 injected with water (Wieler). It is quite possible that in other cases also 

 the conditions necessary to cause bleeding develop gradually, for in many 

 inactive root-stumps an exudation of water may be induced by special 

 treatment. Bleeding is, however, a vital phenomenon, and is not produced 

 by an evolution of gas due to internal decomposition, as Bb'hm ] supposed, 

 although an internal production of gas might easily cause drops of water to 

 be forced out. 



The amount of water exuded varies very much, and it is easy to 

 understand why cut branches often bleed but little. When the process 

 is active, the amount of fluid that escapes soon surpasses the total 

 volume of the absorbing root-system in many cases. Hofmeister found 

 that a root-stock of Urtica urens excreted 11,260 cubic millimetres of fluid 

 in 2^ days, whereas the volume of the entire root-system was only 1,450 

 cubic millimetres. 



Instances of active bleeding. Wieler 2 collected from a hole bored in a twelve- 

 year-old birch 36 of sap litres in 7 days, i. e. about 5 litres per day, while Clark 

 obtained from other species 6-8 kilogrammes in i day. Canstein gives the daily 

 amount escaping from a vine as from 10 to 950 cubic centimetres or more, and 

 according to Humboldt the greatest flow in one day from a decapitated flowering 

 axis of Agave was 7-5 litres (= 375 cubic inches), while during the whole time of 

 bleeding (4 to 5 months), 995 litres (= 50,000 cubic inches) of sap escaped. From 

 a cut stem of Musanga, 0-71 litres of sap were forced out per hour during the 

 night-time, according to Lecomte. 



The sap exudes from the xylem, and especially from the tracheae 

 and tracheides 3 . Bleeding therefore corresponds closely with filtration 

 under pressure, and in both cases, according to circumstances, the escaping 

 fluid may or may not be mingled with air-bubbles (cf. Sect. 36). These are, 

 however, usually absent, especially when the tracheal elements are com- 

 pletely filled with water, although air-bubbles may appear at first if the 

 opened tracheal elements contain chains of air-bubbles and water- 

 columns 4 . 



1 Bohm, Bot. Zeitung, 1880, p. 34. 



8 Wieler, Cohn's Beitrage, 1893, Bd. vi, p. 114. See also Hofmeister, Detmer, &c. ; Meyen, 

 Pflanzenphysiol., 1838, Bd. 11, p. 85; Lecomte, Compt. rend., 1894, T. CXIX, p. 181. [Molisch 

 (Ann. d. Jard. hot. d. Bnitenzorg, n. suppl., 1898, p. 23) collected nearly 8 litres in eleven hours 

 from a single bore-hole in the stem of a liana (Conocephalus).'] 



3 Schwendener, Sitzungsb. d. Berl. Akad., 1886, p. 575 ; 1892, p. 531 ; Strasburger, Leitungs- 

 bahnen, 1891, p. 859; also Briicke, Ann. d. Physik u. Chemie, 1844, Bd. LXill, p. 181, &c. 



* Literature : Unger, Sitzungsb. d. Wien. Akad., 1857, Bd. XXV, p. 442; Hofmeister, Flora, 

 1862, pp. 102, 113; Barauetzky, I.e., 1873, p. 39; Strasburger, 1. c., p. 847. No attention need be 



