SELF-REGULATION 5I? 



All regulatory phenomena arise from stimulatory reactions, the nature 

 of which is still obscure : thus when a plasmolyzed protoplast clothes itself 

 anew with a cell-wall, it is uncertain whether the removal of the pressure 

 against the original wall, or the change in the surface-tension, or some other 

 cause, acts as the stimulus exciting the new activity. It is probable that 

 changes in turgidity act directly as regulatory stimuli maintaining constant 

 turgor l . Similarly the inhibition of the removal of the endosperm food- 

 material is not entirely due to the action of a chemical stimulus, for the 

 same inhibition may, according to Puriewitsch 2 , be produced by corre- 

 spondingly concentrated solutions of other substances than sugar. The 

 chemical character of the substances in the nutrient medium is, however, 

 of great importance in the suppression of diastase formation in Pcnicillium, 

 for certain forms of sugar are especially active (Sect. 91). Chemical stimuli 

 undoubtedly play a most important part in metabolism, and these may 

 arise from normal products or in certain cases from specific stimulatory 

 substances which plants may employ to attain a particular end-. There is, 

 however, no reason for Reinitzer's 3 supposition that specific fatigue- 

 substances are produced by plants in general, for the mere accumulation 

 of a normal product may result in the depression of a single function 

 or of the general metabolic activity. Similarly different organisms may 

 influence one another by means of the metabolic products they excrete, 

 either permanently or only as the result of the action of specific stimuli. 



1 Pfeffer, Druck u. Arbeitsleistung, 1893, p. 428. 



2 Puriewitsch, Ber. d. Bot. Ges., 1896, p. 209. 



3 Reinitzer, Ber. d. Bot. Ges., 1893, p. 531 ; Jager, ibid., 1895, p. 70. 



