264 THE MOVEMENTS OF WATER 



In what has been said so far only the activity shown after a given 

 temperature has persisted for a time has been considered, for a change of 

 temperature produces a direct physical effect upon the rate of flow, owing 

 to the altered volume of the enclosed gases (Sect. 37). A sudden rise or fall 

 of temperature may thus temporarily affect the height of the manometer, 

 owing to the acceleration or retardation of the flow of water which is 

 involved. Bleeding at constant temperatures cannot be due to any such 

 transitory actions as these, as Matteuci and Sarrabat erroneously concluded. 

 Similarly, Hofmeister incorrectly supposed that a continuous exudation of 

 water can be maintained by the pressures and changes of volume due to 

 alterations in the tissue tensions'. 



Light and Gravity. Light apparently exercises some influence upon 

 the escape of water, as is indicated by the daily periodicity of the latter ; 

 but gravity appears to be without effect, for Wieler found that the 

 rate of flow from a plant was the same in an inverted as in a normal 

 position. 



Oxygen and Chloroform. In the absence of oxygen the exudation of 

 water ceases, although the cells may remain turgid. This is the case at any 

 rate in the grass seedlings examined by Wieler, for the escape of water 

 ceased as soon as the air was replaced by hydrogen. That the exudation 

 of water is a vital phenomenon is shown by the fact that it ceases when 

 the plant is anaesthetized by chloroform, as Wieler proved by placing the 

 roots of seedlings, or of older plants from water cultures, in a dilute solu- 

 tion of chloroform in water. The experiments are not always conclusive, 

 since if the chloroform is too strong the plant is readily injured, or may 

 be killed-. 



SECTION 45. The Periodicity of Bleeding. 



The power of bleeding probably varies at different stages of onto- 

 gcnetic development, and like other vital phenomena may be subject to 

 periodic alterations. The older observations rendered the existence of a 

 yearly periodicity probable, especially in trees, but this was first definitely 

 ascertained by Wieler 3 , who by using plants growing in pots was able to 

 obtain constant external conditions throughout the year. According to 

 these observations, most hibernating trees lose all power of exuding 



' Sarrabat, quoted by Dutrochet, Memoires, 1837, p. 199; Mattenci, cf. Hofmeister, Flora, 

 1862, pp. 101, 171. G. Kraus and Detmer made similar conclusions (cf. Wieler, I.e., p. 152). 



2 Wieler, 1. c., pp. 65, 69. 



s W 7 ieler, Cohn's Beitrage, 1893, Bd. vi, p. 72, and Tharander, Forstl. Jahrb., 1893, Bd. XLIII, 

 ]>. 156. The older literature (Ray, Hofmeister, Hartig, &c.) is given by Wieler. 



