THE INFLUENCE OF EXTERNAL CONDITIONS 263 



Ravizza, ibid., 1888, i, p. 68; Hornberger, Bot. Centralbl., 1888, Bd. xxxm, 

 p. 227; A. Fischer, Jahrb. f. wiss. Bot., 1890, Bd. xxn, p. 73; Wider, Cohn's 

 Beitrage, 1893, Bd. vi, p. 158. 



SECTION 44. The Influence of External Conditions. 



Like all vital phenomena, bleeding is markedly dependent upon 

 external conditions, and thus both the flow of sap and the exudation- 

 pressure diminish as transpiration becomes more active. A dry soil exerts 

 a similar sucking action, and may indeed ultimately induce a backward 

 flow of the ascending sap l . 



Saline solutions, again, by withdrawing water from the plant, may 

 cause bleeding to diminish or cease 2 . Wieler observed that in a variety 

 of plants the escape of water completely ceased when i per cent, of 

 potassium nitrate or 2 per cent, of glycerine was added to the nutrient 

 solution in which they were grown, but that after a time the exudation of 

 water recommenced. As the salt is slowly absorbed, the plasmolytic action 

 of the concentrated solution is neutralized, while any temporary injurious 

 action of the sudden change will soon disappear as the plant gradually 

 accommodates itself to the altered conditions. In the process of ac- 

 commodation transitory variations may occur, the origin of which is quite 

 uncertain. When an injured plant which has been subject to drought 

 for some time is watered, it may at first bleed very actively before it 

 assumes the rate of flow corresponding to the new conditions. Similarly, 

 by suddenly increasing the height of the column of mercury which the 

 bleeding-pressure supports, the latter is caused to fall at first below the 

 new level, which corresponds to the changed conditions 3 . 



Temperature. When the temperature is too low, bleeding ceases, 

 even although an absorption of water may still be possible (Sect. 37). 

 Many European plants bleed perceptibly even at o C., but in Ricinns, 

 according to Wieler, an exudation of water is only possible above i to 

 3 C., and in Cncurbita melo not below 7 to 9 C., according to Detmer. 

 The flow rapidly increases as the temperature rises, but it is uncertain 

 whether the rise continues until the temperature becomes dangerously 

 high, or whether it falls again after reaching a certain optimum. It is, 

 moreover, doubtful whether the same relation exists between the pressure 

 and the amount of flow at all temperatures. 



1 Experiments of this kind by Hales, Hofmeister, Flora, 1858, p. 6; Detmer, Beitrage, 1877, 

 p. 34; Baranetzky, 1. c., p. 31 ; Brosig, Die Lehre von cl. Wuizelkraft, Breslau, 1876, p. 25. 



Wieler, Cohn's Beitrage z. Biol., 1893, Bd. vi, p. 52 ; Brosig, 1. c., p. 25; Detmer, Mitth. a. d. 

 Gesammtgeb. d. Bot. v. Schenk u. Liirssen, 1874, Bd. I, p. 4.52. 



3 Cf. Wieler, I.e., p. 49. Gain (Rev. gen., 1895, T. m, p. So) did not observe any such tem- 

 porary rise ; Wieler, 1. c., p. 1 26. 



