THE PERIODICITY OF BLEEDING 267 



a decided daily periodicity cannot be detected in all cases, and it is even doubtful 

 whether the maximum for a given plant always occurs at the same time. Baranetzky 

 did indeed find that the maximum flow occurred at a specific time in some cases in 

 the morning, in others in the afternoon and that, like the growth-periodicity, each 

 specific bleeding-periodicity was comparatively constant. Nevertheless, many excep- 

 tions to this are known, and Wieler has even found that the bleeding-curves for two 

 different individuals of Alnus glutinosa may exhibit entirely different periodicities 

 (I.e., p. 136). 



It is possible that the periodicity in bleeding is induced by the periodicity in 

 certain of the vital processes to which the exudation of water is due, such as, for 

 example, the power of reacting to an injury ; hence the bleeding-periodicity may 

 vary as the causes alter which induce it. It is at the same time possible that the 

 daily periodicity of bleeding, like the daily periodicity of growth and movement, 

 may be induced by the periodic alternation of night and day influencing internal 

 pressure and the turgidity of the tissues. When dealing with the phenomena of 

 growth and movement, a more detailed account will be given of the manner in which 

 the rhythmic repetition of particular external conditions may cause a pronounced 

 periodicity to be induced, owing to the continual summation of the new stimuli with 

 the after-effects of the previous ones. A pendulum may be caused to swing to and 

 fro with considerable amplitude by regularly applying a very feeble force at any given 

 point of its course. When the force is no longer applied, the pendulum continues 

 to swing for a time, the oscillations become less and less marked, but the same 

 interval still intervenes between each, i. e. an induced periodicity may gradually die 

 away again, but its time limit remains constant. 



According to Baranetzky the daily periodicity in the exudation of water is 

 actually induced by the alternation of night and day, and hence it is not at first shown 

 by young plants, but gradually makes its appearance if they develop under normal 

 conditions. Baranetzky found that an alteration in the periods of illumination caused 

 the daily periodicity to change; Brosig (I.e., p. 35) remarked that in a certain plant 

 no such effect was produced ; Baranetzky again has shown that in many cases no 

 daily periodicity at all is exhibited. It is, however, only natural that such excep- 

 tions should exist, when we consider that no two plants are specifically identical. 

 A bleeding-periodicity may be induced in a transpiring plant, although no actual 

 exudation of water occurs during the period of induction, hence it is probable that 

 in such cases a periodicity has been induced in certain other allied vital activities 

 upon which bleeding depends. Moreover, it was in the root that a daily bleeding- 

 periodicity was first observed and measured, although the root is not directly 

 exposed to the periodic alternation of daylight and darkness. 



1877, p. 41 ; Brosig, Die Lehre von d. Wurzelkraft, 1876, p. 3 ; Wider, I.e., p. 129, where details 

 are given. 



