EHYTHM 361 



minimum turgescence in a radially symmetrical organ may 

 lead to a similar nutation. It is not infrequent for the 

 rhythmic change in the turgescence to affect only two sides, 

 instead of passing regularly round it. The organ, though 

 radially symmetrical in structure, will thus behave as a 

 bilaterally symmetrical one, its organisation indeed being 

 bilaterally symmetrical. Its changes of position will thus 

 resemble those of a flattened organ which can only be made 

 to oscillate backwards and forwards. 



A similar rhythm can be noticed in the variations of the 

 extensibility of the limiting membrane which characterise 

 the circumnutation of a coenocytic hypha. We must sup- 

 pose these variations to be due to the protoplasm covering 

 the wall, though we cannot explain the mechanism. The 

 protoplasm has the power to soften the cell-membrane. 



Khythmic changes of this kind affect other processes 

 than those of circumnutation. We have had occasion to 

 notice that the behaviour of a growing organ during its 

 grand period shows certain diurnal variations which we 

 have called the daily periodicity of growth. Though no 

 doubt we have to do here to a certain extent with changes 

 in the behaviour of the protoplasm induced by the alter- 

 nations of light and darkness, with coincident variations 

 in temperature, this daily periodicity of the rhythm does 

 not appear to be altogether dependent upon exposure to 

 such alternations, for they persist for a longer or shorter 

 time during continuous darkness. Their cessation after 

 exposure to a period of darkness need not necessarily 

 point to their dependence on the intermittent access of 

 light and warmth, for, as we shall see later, prolonged 

 deprivation of light leads to a peculiar condition of rigidity 

 of the protoplasm which eventually causes its death. The 

 cessation of the rhythm indeed appears to be a pathological 

 phenomenon. The rhythm of the daily periodicity appears, 

 however, to bear a certain relationship to the alternation 

 of day and night, for plants which have been cultivated 

 from seed in continuous darkness do not exhibit it. 



