254 THE MOVEMENTS OF WATER 



drips away. The water is pressed out with a certain force, as may be 

 shown by fixing a manometer tube to the decapitated stem, or to a hole 

 bored into a tree trunk, and noticing the height of the column of water 

 or mercury which can be supported by it (Fig. 33, p. 2.56). The pressure 

 of exudation l is often considerable. 



Bleeding may be shown by many plants provided they are sufficiently 

 saturated with water. If the accumulation of the latter is due to the 

 absence of transpiration, water may commence to flow immediately the 

 stem is cut across. This is the reason why a vine-stem bleeds when injured 

 in spring before the buds have opened. When transpiration is active, how- 

 ever, the amount of water present in the stem diminishes (Sect. 34), so that 

 in summer the stump of a branch may absorb water at first for a time, 

 and often in large amount before any exudation begins. The latter never 

 takes place so long as transpiration is active, but only when the absorbed 

 water is allowed to accumulate. 



The fact that bleeding is possible in summer when transpiration is 

 prevented was first recognized by Hofmeister, while the phenomenon of 

 bleeding was first observed by Ray, and studied more in detail by Hales. 

 The latter observers used trees only, but Hofmeister showed that bleeding 

 was possible from herbaceous plants as well ~. These authors and others who 

 followed them considered the active exudation of water to be due to the 

 pressure cau.'-ed by the absorptive activity of the roots, and positive results 

 were indeed first obtained with root -stocks and with single roots or even root 

 apices 3 . Many aerial stems and other organs, however, possess a similar 

 power of bleeding, as was first shown by Pitra, and confirmed later by C. Kraus 

 and Wieler 4 . Whenever water is driven into the vessels or intercellular spaces 

 by the active living cells of the stem or even of leaves, a pressure sufficient to 

 cause a more or less marked exudation of water will be produced <r> . 



If leafy branches of Finns sylvestris^ Qnercus robur, Primus cerasns, &c., 



1 [The term ' root-pressure ' is obviously incorrect as applied to this general phenomenon, while 

 ' bleeding-pressure ' suggests an incorrect analogy with the arterial pressure of animals. ' Sap- 

 pressure,' on the other hand, is extremely vague, whereas ' exudation-pressure' or ' pressure of exuda- 

 tion 'seems to meet all requirements, and may therefore replace 'root-pressure.' The latter term 

 may, however, still be retained to indicate the osmotic pressure of absorption which may be generated 

 in the root when immersed in water or in a dilute nutrient solution. Root-pressure is therefore 

 a physical, not a vital phenomenon.] 



2 Hofmeister, Flora, 1858, p. i ; Ber. d. Sachs. Ges d. Wiss. zu Leipzig, 1857, Bd. IX, p. 149 ; 

 Ray, Hist, plantar., 1686, Vol. I, p. 8; Hales, Statics, 1748. 



3 Researches of this kind were made by Dntrochet, Me"moires, Bruxelles, 1837, p. aoi ; by 

 Dassen, F. Froriep's Neue Notizen, 1846, N. F., Bd. XXXIX, p. 133; and also by Hofmeister, 

 C. Kraus, &c. [Cf. H. S. Chamberlain, Bull. d. 1'universite" de Geneve, T. n, 1897, p. i.] 



4 Pitra, Jahrb. f. wiss. Bot., 1877, Bd. XI, p. 437; C. Kraus, Flora, i88a, p. 2; 1883, p. a; 

 Forsch. a. d.Geb. d. Agriculturphysik, 1887, Bd. X, p. 67 ; Wieler, Cohn's Beitrage, 1893, Bd. vi, p. i. 



s BrUcke (Ann. d. Physik n. Chemie, 1844, Bd. LXIIT, pp. 203, 206) first called attention to 

 this fact. 



